Engine swap advice

possible bad news the last two slant 6's I have had apart was because the unleaded gas had started to beat the valve seats into the heads. what happens is the valve to seat has no lubrication and the valve works like a hammer on the seat until it eventually beats enough that it doesn't seal anymore.

do a compression test on the 6 and see if this is your issue.
if you do a remodel of your '63 to fit the V-8 you might consider picking up a '64 or newer firewall and paste it in so you still have windshield wipers etc. it is better than the sledge hammer solution.