I for sure have a set of J heads that were on a 340 of mine.
I have the original 302 heads.
I also have another set of heads that were on a HFT 318 motor, not sure the casting number though, I'll have to look tomorrow.
Okay, the 302s obviously are a bolt-on, and have the correct combustion chamber size to keep a minimal compression ratio with your stock 318 pistons, which the factory listed as 9.1:1. In actuality, they're probably in the high 8s.
The Js will need the pushrod holes clearanced, and have a larger combustion chamber that will likely drop your cr into the low 8s if you're lucky, and depending on what head gasket you use. Unless they have been cut... then it's dependent on the cut.
Your other 318 heads will need the pushrod holes clearanced, the combustion chamber is an unknown since they can vary quite a bit depending on year- early ones (don't know the numbers offhand) have a small combustion chamber, but no hardened seats (if that matters to you) and later ones (early/mid 70s?) started getting the larger combustion chamber, but had hardened seats.