Which 273?

I think we concur that the initial post giving casting numbers for the block is irrelevant since the tapped holes for the intake manifold are in the heads. Any small block heads can bolt to any block. You could even bolt Magnum heads to your 1965 block, but then would have to use a Magnum intake, which may be desired - cheap way to realize multi-port fuel injection. I understand the stock valves will clear the 273 cylinder walls. Downside is that Magnum heads are known for cracking at the valve seats (no inserts, induction hardened). No comment on posts 18, 21, & 23.