can you verify with certainty that the block wasn't decked? a lot can happen in 50 years...
i had the same problem that i chased my *** on-- even so far as changing the heads and intake out. turns out the block had been decked on a previous build.
prior to the guy owning the car the PO had the motor built and even though it had a refresh before the problem cropped up neither he, nor the current owner were nuts and bolts guys so that wasn't something that was on their radar. to make matters worse, the machinist/engine builder had long passed his use or freeze by date. all the paperwork i had from the work done was a cam card.
anyway, a hearty +1 to motorcycle/powersports sealants. motoseal, threebond 1184, hylomar are all good choices.