318-3, J heads, Hardened seats, and a few other ????'s

I think you can pull the numbers off the side of the block to determine what it is.

I do not believe 73 J heads had hardened seats in them and even if they did I would get them redone.

Bolting on those J heads to your 318 will probably drop compression a full point. They probably have 68-72cc heads. While 318 heads had 61-64cc heads from all I have measured.

I would not risk driving the car without hardened seats, I can not inform you of the risk, but I do know there are.