Welcome to CHR!

Can you tell us a little bit about the build?
1. Is the 350 stock (4.000) bore?
2. Have you measured the block to make sure head mating surfaces are square (to the crank) and flat?
3. Has the block been surfaced?
4. Has the block been cut? (Stock is 9.025”)
5. Are the heads stock? (And why are you using 50 plus year old heads? They typically have small chambers for the high-octane fuel readily that was available at the time they were used by GM. Are they 1.94 intake/1.50 exhaust or 2.02 and 1.60? Finding a good set is not easy and you'll need to have them magnafluxed bofore use.)
6. What pistons are you using?
7. What cam geometry are you planning on using? (i.e., duration @ .050”, rocker arm ratio(s), and lobe separation
Fill in the blanks and we can certainly help you out.

Best,
Glenn