high compression

Number 5 either has a popped head gasket, valve going, cracked ring or notches are lined up, in that order. You never did tell us what stroke, 4.00, 3.85, 3.75, 3.58 and Bore 4.04, 4.07, 4.08. The standard 340 with the slight dome, 4.04 bore, 68cc chambers is 10.5 to 1, with a same pistons and a 4.00 crank is a 13.5 to 1 or 14.0 to 1. So we need to know 0 deck or not, flat tops or dome and if domed what type and shape, stroke, heads, and ring position or type of piston. But I would bet on head gasket looking at the numbers (Between 3 and 5), either way heads are coming off. I wouldn’t play with it take them off and look, the last thing you want to do is hydraulic it, then its junk. If it is the head gasket resurface both heads and block.