I have a RB25 180sx and the dry weight was 1245. A standard SR 180sx is around 1200 I believe. Add to this I have 18"s then this whole 'wreaks handling' arguement is way overdone! I'd suggest you wont notice any difference unless you are on a fast race track. As for the weight balance question in general, there are plenty of front wheelers out there that handle with the best of them despite 65/35 distribution. As people have mentioned, it's all about correct setup.
Anyway why not consider somthing different such as a 1jz? I'm actually selling my RB25 180 to do just that. Starting with a NA silvia chassis then adding a TT model 1jz, w58 manual gearbox and the icing on the cake, a Blitz k26 turbo kit from Japan, plus alot more goddies. You can get a 1jz halfcut (auto) for $2000-$2500. The 1jz is highly underrated, I suggest just as durable bottom end (300rwkw+) as the rb26 but maybe not as efficient topend (cam's, intake etc) in standard trim. Stll you can have four 1jz engines for the price of one rb26......
'Nistoy' cooming soon