I don't know exactly, but it seems like I read on the forum that the stock B4 shock is 312.5, I tried to find the post, but couldn't. I am at VIR today for a track day, so I don't have access to measure the old one with any accuracy. According to Ohlins, the GSX-R '96-99 is 345mm, '00-03 is 325mm and the '04-05 is 332.5mm.
BTW, are you going to use your GSX-R front end? If not, I may like to buy it.
Later,
RacerRuss