Why not wait until you bolt the heads on and eliminate any guess work?
Felpro offers 2 head gaskets. Here is what info I have on them from Rocky Rotella's Pontiac book:
Fel-Pro introduced PN-8518 for the Pontiac V-8 during the 1970s. Containing a bore diameter of 4.3 inches and a preflattened thickness of .041 inch, itís a stock-replacement design thatís compatible with most Pontiac blocks. an affordable choice for any stock rebuild, and even performance applications with a compression ratio up to 10.5:1, or slightly more.
A second Pontiac offering appeared in the Fel-Pro catalog during the early 1980s. PN-1016 is specially designed for high-performance applications with high compression and/or combustion pressure. Constructed of a Teflon-coated solid-steel core, it features stainless steel combustion armor and a wire ring combustion seal for maximum combustion containment. It contains a chamfered bore diameter or 4.3 inches and pre-compressed thickness of .039 inch. itís a good choice for naturally aspirated engines with a maximum compression ratio of 12:1 and/ or mildly boosted applications.
These gaskets, he says, have larger than factory water passages and are close to the sealing ring which could be a problem IF
the seal around the cylinder were to let go.
The book states the Butler offers a composition gasket, .045" when compressed, that has the factory sized water passages. Butler recommends a light coating of copper sealer on the steel side for maximum block seal. Good for engine up to 11.5:1 and mild boosted engines. https://butlerperformance.com/c-1234...d-gaskets.html