It would help to know your year and trim level. Your best bet is to get the complete rear end off a GSL appropriate to your series, eg if you have a Series 2 then get the rear from a Series 2 GSL. You can also buy the complete front and rear suspension from a GSL-SE and it will bolt right up. This nets you an improved suspension plus a wider selection of wheels as the SE has a different (more standard) bolt pattern.
Putting second gen hubs on a first gen requires a lot of changes if you go with the five bolt units. There is a company that offers kits to permit this, but you have to buy all new rear axles. The four bolt second gens have the same bolt pattern as the SE trim, so it would just be easier to use the SE components as I mentioned above.
I don't know of anyone at this time that offers a kit to replace your rear drums with discs. There has been some talk of it being done but nothing has come of it.