JB weld, flex seal or full rebuild ?

Was the block sonic checked when you built it the first time? I would get that checked first if it hasn’t already been done if it looks like it’s going to need bigger than a .040 over piston. It’s dependent on year, but sometimes going much past .040 on certain 360 blocks can make things thinner than they need to be. If it’s a ‘71-‘73 360, though, the odds would be more in your favor.