yeah, i did some measuring.... i pulled them off and swapped them over
the engines/frames are VERY similar on these two bikes. i've managed to pinch a bunch of parts off the 91 to replace broken/bent bits on the 99

so yeah, thought it was worth a shot, looks like i was right. the studs on the 99 are a little shorter so it was a little bit of task to get the new headers on but everything else fit like a glove.
so now it looks a little better, sounds a lot better and i've gotta get used to a bike with a completely different power curve now. it obviously needs a tune, but i've got a very 'soft' feeling bottom end now (idle to 3-4k) the middle feels a little more solid and the top surges like theres a firecracker lit up its arse
anyways, cheers for the advice as usual