Someone in WA has done the swap, he says it's bolt on even the gearbox, will need to clean or swap dome of fuel parts (clean lines, tank might need to replace fuel gauge sender).
Look up Patrol forum ( patrol4x4.com I think, google will find it) I'm sure there are heaps of information regarding this kind of swap.
iplen, OP is buying a GU which never come in RB30 the one you were talking about is GQ which comes with RB30 (I used to have one myself along with ET conversion).
As for getting a mechanic to do the job, just go to whoever you trust to have it done.
Hope it helps.
Cheers