I'm rebuilding a '75 350, block cast # 395558 2 and the heads cast # 411929 8 and I have '77 heads cast #554718 3A, What heads would be best to use?

 

The later heads are even worse than the #8 heads. On top of that you would need to use inserts for the head bolts because 77 and up blocks use ½" head bolts and the early ones use 7/16".

 

Good read on head info- just the facts.

http://www.442.com/oldsfaq/ofhed.htm

Couple things there- What are your power expectations and what does "rebuild" mean in this context?

#8 heads have 79cc chambers, with flat top pistons and a lot of machine work you'd just inch inch over 9:1 and still have smog heads.

The parts section in CO is full of people near giving away #5, 6, 7a heads that might be better for your application. Even if this is a mid-low compression budget rebuild every engine likes to breathe better.

 

If you are going with new pistons, #8 heads with flat tops are a no brainer. Just mill the small amount to get 9 to 1, add a 2" or 2.07" intake and reuse the factory 1.625" exhaust valve. The only bad area on #8 heads is the lip under the exhaust valve. Easy to remove, just a pocket port on the bowls, intake to match the larger valve and you will have a head on par with early heads. 3A heads are crack prone, restrictive junk with an awful exhaust port.