GTR to rwd is easy enough, drop front driveshafts, hack front diff off sump, remove attessa computers etc and chuck in an RB25 box. But I don't see the point.
Have you driven an R and then come to the decision that you don't like the AWD or have you just gone, "Yep the GTT is rwd I like it" and not done any research into how the R actually handles.
I haven't driven a 34R but I have driven a 33R and whilst the 4wd and rwd feel completely different it is a great handling package. Feels a bit heavier in the front but it feels a lot more balanced and smooth. If you want your GT-T to run an rb26 then do that and save the coin on everything else. otherwise drive an R if you haven't already and see if you really do dislike the AWD as much as you think you do.
Another thing to consider, as well as the guards being pumped out and the suspension being mostly the same you will have to swap rear hubs from loop type suspension to fork type and LCA's etc are different to provide you with the added 20mm rear track. Front track however is the same.