I pretty sure the steering stem, which is pressed into the lower tree, is longer on the later model GSXR's, so you'd need to press the stems out of both the B4 & GSXR and press the B4 stem into the GSXR tree. Be careful! The stem isn't real sturdy and will bend if you aren't careful. Best left to someone with machine shop tools & experience if you aren't so equipped.
BTW, you'd still be using B4 bearings this way.