Not to slander pishta, but, I wouldn't use 315 heads. They're even more crack prone than the 920's and you have to do the mods to the intake to get them to work. There are much better heads out there. Keep in mind that if you change to another casting number, you may change the compression ratio. In some cases, drastically. Also, the later smog port heads will need to have plugs installed in those ports. or you will have exhaust leaks. You say you have headers, so, there should be no exhaust interference that stock manifolds have on smog heads. Should you change back to stock manifolds, you'll need to grind part of the head casting on the driver's side or you will break ears off the manifold.
