I love my b18cR with the 4.78 FD, but couldn't imagine a 4.92 or otherwise. If I were to go the 4.92 route I would have to throw in an LS 5th and retain the other 4 gears for ripping the pavement up .. or not haha.
i got a quote to fit the mfactory 4.9 final drive for £300 man, gearing company in scotland cant remember the bloody name though as it was like 2 years ago lol, but shop around that is quite high