There can be just as much adjustment on the BMW stuff if you want.
Diff ratios are cheaper ad easier to find too but as you have everything already you might as well make use of it. Skyline rear end will probably give better ultimate grip too.
As long as you don't get carried away and fit everything from the skyline and weld it on crooked

As mentioned too, shove the engine right back into the bulkhead. Friend of mine had a RB engined E30 a few years ago and it was kinda shit really although it only being a RB20 wasn't helping matters.