Most rear disc brake conversions are boy-racer garbage. The big drums brake very well, but only when kept in proper adjustment, which is a pain in the butt.
Supposedly the 14-bolt JB6 brakes are the best factory combination, but I haven't stepped up to them on my truck yet. I spy five lugs, and unless you have a 454SS rear axle, you don't have them either. If you want to go that route, I'd get a later 14-bolt rear axle and get some five lug axles or have the six luggers redrilled.
Always check the friction code on your pads. You want GG or HH. A lot of "performance pad" companies recognize there's a huge market of people who don't know brakes, so they sell "performance" pads with an FF friction code. They'll last a long time, won't make too much brake dust, and won't stop worth a flip. Real performance pads wear out quick, make lots of dust, and threaten to kiss the pavement with your front bumper.
Raybestos Element3 brakes are dirt cheap and the guys who have them report an HH friction code. Guys I know who race heavier cars, Crown Vics etc., love Raybestos brakes because they're 1/3 the price of "performance" pads, brake hard, and work reliably lap after lap on track.