I never felt under braked with the drums, but lets face it discs all round is way cooler, even to the extent that I painted the drums silver so they looked like discs.
You will still need the spacers and I think the adapter plate takes the place of the last spacer.
The real problem will be sourcing the parts from a xr4x4 or cossie sierra unless you go for aftermarket (or someone kind stripping a sierra rear subframe for an LSD gives them to you (how I got them) or someone kind finds them in his bit box and gives them to you (how I parted with them)).
If budget is a consideration you will not be disappointed with the performance of the drums and it may even be easier to get the right brake balance as discs (I believe) can overbrake the rear and lead to embarrassing end swapping without a (fixed for IVA) bias valve.
so quite a shopping list for a disc braked rear.