New T stat & new radiator

I am going to replace my old, rusty radiator in my '69 Goat.  Any suggestions?  The engine is pretty much stock.

I am also going to replace the thermostat.  I want to get one that runs the engine a little cooler so the chances of overheating and causing damage is more minimal, but I want an efficient burn.  160* or 180*??  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

I would go with a 160.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Okay, I'll get a 160*.  I am thinking a Summit aluminum radiator.  Anyone had any experience with these?  I trust Summit's products.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

A friend of mine bought one and is happy with it says it works great but have never used one personally.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Oh good, good.  I ordered the radiator, t stat, header wrap, starter heat shield, and heat tape.  I also have an elec. fan to install.  It's going to run much cooler and be much happier, and so will I!  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

I drove the car to work today.  It did pretty well.  It's still got a little hesitation issue, a little stumble, but when I hit the gas hard it goes.  The temp did not exceed 160*!  I'm very happy with that.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

Glad to here that the hesitation could be the timing or the accelerator pump not adjusted properly.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

With my vacuum gauge hooked up, at idle I have a rapidly vibrating needle around 13-16 in Hg.  If I advance or retard the timing from where it is, the needle will drop lower and continue to vibrate.  The A/F mixture screws are about 1 turn out.  If I rich or lean the mixture, the needle will drop and continue to vibrate.

Accelerator pump??  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

A fluctuating vacuum needle could be caused by many things. Erratic flux is indicative of timing or mixture. A uniform rhythmic vibration is indicative of other problems, i. e. valve problems.  
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Good Judgment comes from Experience...Experience comes from Bad Judgment

This is interesting:  I have a new PCV valve in my valley pan connected to the Holley carb for vacuum using a section of hose.  Using a long screw driver placed on the hose and held to my ear, I can hear the PCV valve rapidly opening and closing.  If I use pliers to squeeze the hose shut, the idle drops and the vac needle drops indicating less vac and continues to vibrate.  What does this tell me?  This is not normal, correct?  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

this sounds too strange to diagnose without looking at it. The PCV is obviously pulsing due to the vacuum signal, which makes sense. If I had to guess I'd say the PCV vacuum line is feeding your carb extra air. when you have the idle mixture adjusted with this extra air it will be too rich when you pinch the tube. The idle will drop. But your fluctuating needle is being caused by something else. If I were you ( and this is without seeing it) I would do a running compression check. This will tell you if your valve train is operating correctly. It is possible to have good static compression and fail the running test.

To do this test let the engine warm up. Remove one spark plug and install the compression gauge. Ground the plug wire and start the engine. release the pressure from the gauge and it should return to between 30 to 60 psi. Repeat for all cylinders. They should all be the same but this is not as important as on a static check. If you have a sticking valve or broken spring you will see 0 psi. There is no mistaking it. You will know which one is bad !

I have seen springs brake and not make the slightest noise, give great static compression but make the engine run like crap.  
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Good Judgment comes from Experience...Experience comes from Bad Judgment

Wow that's very interesting.  I will have to try that running test.  Hmmm sticking valve?  I have noticed that there is very little oil coming through the push rods, even while revving the engine.  But, I have ample oil pressure.  Is this related?

I have some other questions.  Could really badly fouled spark plugs cause the vacuum gauge to act this way?  I have a stock spread bore (4 hole) intake manifold, a 1" open plenum carb spacer and a 650 spread bore Holley on top.  Could the open plenum spacer cause this somehow?  Also, I cannot find a leak in the intake, but is it possible that internally, where the intake meets the head, the gasket could be blown between adjacent cylinders?

Thanks!!  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

I just performed a running compression test.  I have 80-90 psi in all cylinders.  All the spark plugs were badly fouled black and sooty something awful.  Except  numbers 4 and 7 looked better than the rest, and number 6 actually looked like a normally worn plug.  So, I have static and running compression.  Now what?  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

If the running comp check is good then you can eliminate anything mechanical, which leaves fuel and fire.
Badly fouled plugs would definitely cause the vacuum flux. Have you checked the float level? Are you sure the needle and seat are sealing? In other words is there any fuel leaking into the manifold through the carb float bowl vents? Again without being able to see it it’s very hard to pin something like this down. Personally, I don’t like the Holley spread bores. Is there a reason why you chose that? What main jets are you running? Is the power valve good? A blown power valve effectively dumps raw gas into the manifold. You can get a blow proof one from Summit.

If you had a vacuum leak that could also throw things off but I don't think that is the cause.  
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Good Judgment comes from Experience...Experience comes from Bad Judgment

You're absolutely right, mechanical failures are eliminated, fuel and spark are left.  The ignition system should be perfect.  New wires, Pertronix ignitor II elec ign conversion with matching coil.  Only problem is the fouled plugs, which is most likely caused by the (drum roll) brand new 650 Holley!  If I could do it again, I would buy a Q jet.  I bought the Holley because it was relatively inexpensive, but now I understand it's a poor imitation of the famous Q jet.

I took off the carb today, and there was fuel dripping out of what seemed like everywhere.  The interior of the manifold appeared wet as well.  Probably in part because I was tilting it, but at this point, I am convinced the carb has some fuel delivery issues.  I only have the mixture screws 1 turn out.  If I give it more fuel it gets pissed off.  I also keep finding the oil level rising on the dipstick and smelling like fuel.  This is all adding up now.  How do I begine to diagnose the carb?

Thank you so much for the help!!!!  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.