New 428 crank or machine my old one

I have a 75' Grand Prix LJ 400.  I got my hands on a 69' 428cid w/4 bolt mains.  The motor has a spun rod bearing.  Should I have the stock crank machined or would it be cheaper to replace.  I'm being very cost congis, but I dont want to make a dumb mistake in the name of saving a penny.  I also was thinking I would use the heads off my 400.  any idea what compreshin that might give me?  

Check with a machine shop for a price nad have your original crank inspected to see if it can be saved before you decide. If it can't be saved, Ohio Crankshaft has a nodular iron replacement 428 crank for $295. Their website is   http://www.ohiocrank.com/home.html
Check it out.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

which heads are on your 400?  

The machine shop gets $200 to check & grind a crank.  I have 5C heads on my 400.  the factory rating on my 428 was 370hp and I'd like to get a little over 400hp out of it after I'm done.  I have no plans to ever race it but the 75' GP is a heavy car and just want it run like people think big clasic american cars should.  

Assuming you are having the whole engine rebuilt, with new oversize pistons, I would go with the new crank and have it all re-balanced. This will ensure that it will last a long time if properly looked after.
A .030" over bored 428 is 432cubic inches. Assuming your 5C heads are the original from the 400, they will have 101cc chambers which will give you 8.6:1 compression ratio, ideal for cruising on 87 octane gas. For a camshaft, I would go with something like the COMP Cams XE262H, I've included the address to the page below. Don't forget to use Shell Rotella T 15w40 Diesel oil as per COMP's recommendations, it will protect the new cam thru thr critical break-in period and there's no reason not to use it all the time, I use it in my Firebird and my Safari van (family hauler daily driver).
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Steve
My 428 is a standerd boar and I have an edelbrock performer rpm cam (ram air 4 specks) and a edelbrock performer rpm intake and a edelbrock 800cfm carb.  Do you think I'll be close to my 400hp goal?  What do you think my compreshin might be?  Lastly I need a piston (CAST) standerd boar any ideas?  My machine shop guy is'nt having any luck finding anything but forged.
Thanks
Mike  

Mike, in my opinion, the Edelbrock cam you have is way too big for what you claim to want from your engine. The 370hp 428 came with 10.5:1 compression and the 068 cam (smaller than the one I listed). The cam you have is for a motor operating up to 6500 rpm and you say you just want a powerful classic (and heavy) Pontiac that won't be raced. You need to focus on torque not hp, especially with the highway gears I'm sure your 75 GP came with. A heavy car with highway gears will be a dog with the cam and intake you've chosen, especialy if you go with the 5C heads (8.5:1 compression on a stock bore 428). I would definitely go with the smaller cam and an Edelbrock Performer (not the RPM), the carb is OK, maybe a bit large but useable.
Another point to consider is this, you have a 40 year old motor that still has it's standard bore and has obviously been abused or poorly maintained (spun bearings!) and your machinist wants to put new pistons in a bore that can't possibly be straight and round? Time to look for another shop! For a rebuild to be reliable and long lived, everything MUST be straight and round, putting new pistons in a bore that is tapered or out of round is false economy and will result in a short lived rebuild that doesn't perform to it's potential. Look at it this way, you are already buying pistons and rings, you only have to pay to get it bored (here in Canada it's under $40/cylinder). Besides, what is wrong with forged pistons? They are considered to be an upgrade from cast due to their greater strength in a performance application.
Remember, this is my opinion, but you won't have trouble finding others on here who will give you similar (good)advice. We don't profit from you following our advice, but the hobby profits when good advice leads to Pontiac owners being happy with their cars. If you take poor advice and the engine is a dog, you won't be happy, then your builder will say "well that's what happens when you don't build a CHEVY!" See where I'm going?
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Hey Bud there are lots of good choises for pistons. check out the on-line catologs and you will find cast, hyperutechics, and forged by several good folks. A "rebuild" without new pistons and a torque plate is a waste of gaskets and spray paint. And like the man said torgue is what you need not HP. Give me 500 ft. lbs. and 350 HP any day.  

Thanks for the info.  I know what you are saying is probly right its just not what I wanted to hear.  My cheap little rebild is going to start getting expensive.  I have 2 young kids and money is tight but I dont want to make a dumb mistake that will cost more in the long run.  When I said I would not race my GP I only ment at the track.  I'd love to have a real fire breathing monster of a GP and tell any one who asked "Thats how all old pontiac's run."  The most important thing to me is driveabilty.  My machine shop has been great!  There only tring to work with what I ask of them.  I'm probibly driving them insane with stupid qustions and cheep sugjestions.  The only thing I have agenst forged pistons is the price.  I was only looking to replace one piston that someone sold one out of the moter. Have I told you Im cheep? If I got the edelbrock heads (87cc I bleive) and went 30 over what would be my ball park compreshion and do you still think the cam would be to big?
Mike  

With flat top pistons and 87cc heads you will have approx. 9.5:1CR
If you are talking about the .470 lift edelbrock cam then no it wont be to big with the 9.5:1CR but there are a lot better choices then the edelbrock cams they are on the lazy side.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races