I've fitted one to a GT LTS, it's a bit fiddly but it works the most important requirement is a large flat dropout so the A2Z plates can be clamped around it.
Check out my MCM980 in my sig as there is one on that.
If i tighten it very tight then it is awkward getting the rear wheel out - but it can be loosened anyway so no real problem unless you get a flat and are looking for a quick change. This could just be my new wheels as the old ones seemed fine :?
There is also some slight flexing in the mount when braking hard but again this has not been a problem for me as the front does most of the work anyway. I would rather this flex than push the flexing into the rear triangle.
Finally I was surprised at how well it all fitted together when i built the bike up - it was easy and i used an ISO to post mount bracket and the caliper then fitted the disc perfectly
what you actually need is a 'sharkfin disc adapter' that specialized produced to convert your v'brake mount into a disc mount.
these are seriously rare/almost impossible to come across these days.
there are alternatives (i.e. the A2Z adapter - but this only works if your dropouts are horizontal) FSR dropouts aren't, so it will fit at an angle and you may get some movement if you brake a roll backwards.
i have an alternative adapter (see image below). all you would need is a longer skewer to fit it (i've been told you can get them for tandem bikes or bikes that require them fro trailer attachments)
this link will show you loads of alternatives that people have come up with (one involving an easy mod to the A2Z adapter): you may need to register to see the images though -
you should also check out the FSR thread on this forum in the '1998---> section' (there is a lot of info to search through, but you'll find lots of it really useful)
upload pics of your bike to this thread and you'll soon know everything you need to know about it
check out my build thread in my sig below - i have a 99 FSR ELite.