350 upgrades

I would try to find a pair of 94cc 6X heads. you can tell the cc by the # stamp on the side of the head.  #4is 94cc,  #6 is bigger not sure of the cc, then the #8 is the largest. I belive the #4 head was used on the 350HO motors. Pinkpaul.  

I would keep looking for a set of #13, #16 or #670 heads as discussed earlier.  They have the big valves and small combustion chambers (72cc) for higher compression and more power.

I paid about a thousand bucks to get my #13 heads rebuilt.  I had everything replaced including the valves and springs.  I also had the heads milled slightly and a 3 angle valve job done.  
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.

Thanks for the input.  I may have located a set of #670s locally.  now all i have to do is convince the wife that i NEED to have those heads to be happy in life.  wish me luck.  

Ya putting 6x heads on the 350 is a bad idea.
Good luck on the 670 heads.
Did you swap out cams yet.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

In response to Pinkpaul, the 6X-4 never came on 350HO's. Pontiac didn't start making 6X heads until late '75, at which time they did use them on 350 engines (both 2bbl and 4bbl) until the end of 350 production in 77. The 6x-8 was used on 400's (again 2 and 4bbl) until 78 while the 6x-4 was used on the 220hp W72 400 in 78 and 79 for higher compression.
Pontiac only made the 350HO for 2 years, 68 and 69. The 68 HO used #18 heads with 1.96" intake and 1.66" exhaust valves, the 69 used #48's with 2.11" int and 1.77" exh valves, both heads produced 10.5:1 compression ratio. Any 350 with 6X-4 heads has 2.11" int and 1.66" exh and 7.6:1 compression ratio.
Obviously putting the later heads on will be a downgrade compression-wise, although most d-ports flow fairly similar numbers port-wise, given equal valve sizes. I have 5C-4 heads on my 350 (early 75) which are functionally identical to 6x-4 heads and they work fine (especially on cheap 87 octane!) but given the chance, I would Love to put some 48's or similar heads on it to try! Basically, don't spend money to get 6x-4's if you can get better heads for the same coin.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Well said.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

I'm planning to build up a 455 w/ 6x-4's this summer, but in the meantime I'm going to put a 350 in my 'Bird so I can get it back on the road. I have a 406 in the car now, but it needs to come out for a forged rod upgrade. I'm using the ported no.48's off the 406, as well as same exhaust, carb, intake- should be nice. I already bought most of the parts I need for the overhaul- new Crane 272H cam, oil pump, timing chain, lifters. It's already been bored .030- just need to rehone and rering it, check clearances and put it back together.

Geno