Just because it fits doesn't mean it's right or worthwhile...
I noticed the extra weight in my ke70 going from a 4k to a 3tc, can only imagine how horrible it would be with an rb20...not only is the weight going to be more forward of the crossmember, but they are also quite a tall motor...the conversion would most likely involve spacing the cross member and totally ruining the suspension geometry...
Rb20's are near on indestructible and sound semi reasonable for a Nissan motor...But about the only advantage they have...