Lowering a 72 3/4 ton

[SnS C10]

i have a 72 3/4 ton gmc truck. it has 8 lug drums all the way around with stock drum spindles up front and leaf springs/coil spring shocks out back. i would be putting 5 lug rims on the car. it will be driven around town and will not be hauling at all. what do i need to do about the brakes? do i need to swap out the whole rear end? in front i was just going to convert to disk brakes. im not sure about the rear though. i was going to do 2.5 inch drop spindles and 1 inch front coil springs for the front. i want a 4 inch drop in back with 3.5 in front. can you guys help me out with the rear end and the issues with lowering and the brakes. thanks