340 upgrades.

Id cheat. If you plan on upgrading in the future, and jus twant to cruise and have some fun for now, I'd run either the "302" later model heads, or the '68-'71 318 heads (smaller chambers). If you use the MP thin head gaskets, and the smaller chamber heads, the compression you gain will far exceed the top end power they may or may not loose. I did this on a late model 360. With the XE268 cam, 9.8:1 compression, and minor bowl work and gasket matching for the larger intake ports, it pulls quite well to 5500, idles at 800, and runs on pump gas. The 302s are the best choice IMO, as they have the unleaded seats in them. If you stay with the factory J heads, I'd run an XE262, and follow Don's ideas with regard to timing chain, springs, and seals.