Drum brakes work very well if adjusted correctly and not over heated for a daily driver.
If going to disc brakes, there are a lot of inexpensive kits out there that use the same metric caliper that came stock on the front of all g-bodies. This is not a bad thing, but you will loose the emergency brake provisions. Depending on where you look, the metric caliper will have a 2.375" diameter piston.
Some of these inexpensive kits will come with calipers that have emergency brake provisions build into the caliper for a little more money. I believe the rear calipers are early to mid 80's Seville calipers. You will need to figure out how to hook up your emergency brake lines to these calipers. These calipers usually have a 48mm piston diameter or about 1.88"
There is the 4th Gen f-body rear disc conversion. The only downside to conversion is that you will have to modify the axle flange for it to bolt up, the emergency brake mechanism will have to be hooked up to it, and the piston is on the small size at about an 1.77 or 45mm.
