I'll chime in kinda half assed here, but virtually all the other Mitsu 4G6X engines from a later model years are interchangeable between other years. You can swap heads of many model years between the two.

Between the G64 and G63 of the 8 Valve variety, I'd be willing to bet they are interchangeable. The deck castings are the same, the timing is the same, and pretty much all the other componets are the same. The only thing I would double check is the manifold patters and combustion chambers. Your compression ratios may change. The G64 doesn't have the jet valve either (no big deal).