There are many differences. The GTi-R is very different from all SRs period. The head is unique to the GTi-R, it has solid lifters, quad throttle body, and has a compression ratio of 8.3:1 instead of the 8.5:1 that all other DETs have. So to make it short, nothing is interchangeable between the GTi-R SR and the RWD DETs other than very minor things. As a matter of fact, the RWD SRs don't share too much with their AWD/FWD brethren.
There are other kind of DETs too, other than just the GTi-R. I recommend that you use the search button and look up the archives.
the bottom end is interchangeable for all SR's.....its mostly the heads that are different between sr's. the reason for the gti-r's lower comp ratio is a fully domed combustion chamber on the head, instead on the squared off tangents on the 53j head (most common fwd head). Also, make use of the search button.....there are 3 or 4 rwd DE(T) engines from silvias, and countless fwd/awd applications (dont be scared by awd, its just a transmission...yours bolts on)