Can i ask why?
If you've got nothing better to do and cash burning a hole in your wallet, all the power to you and go for it.
On the other hand, if you want something specific from the car - track car, drag car etc etc, what is it about a 2j that you think the RB can't handle/do?
Option 1 keep RB and spend money sorting it
Option 2 pay money for 2j, pay for transplant and still have an engine no where near as good as option 1 for the same coin.
I just don't get it?
Oh and yes i live teh RB, but my thinking would be the same the other way around. Would never consider puutting an Rb in a supra. Not because they're not good, but because the 2j is fine.