Going from FWD to RWD requires a lot of changes, such as swapping over the LIM and UIM from the f-body onto the new engine, you'll need the fuel lines, injectors, etc. from the old, basically the whole top end. The long block stays the same, balancer can stay, etc. all the accessories must transfer over. The oil pan and pick up tube also need to swap over.

The series 3 engine (L26 if NA), has little change over the L36. Biggest things are cracked rod caps, some of them had ever so slightly larger valves, heads have MUCH nicer ports in both exhaust and intake runners, heads are stronger (better casting), there's a few other things like the oil galleries in the timing cover are better done. Swapping one in requires NO tuning, the difference is negligible. Not to say a good mail-order tune would be a good thing for it.