another question, before you shell out some cash, is whether you will be sticking with the stock rear axle. since it is a tad older you may want to go through it and replace what is worn out, bearings, seals, check splines on axles, wear on gears etc. it is probably not geared all that well for today's highway speeds and if spending money you may want to think about if you are swapping the whole rear axle to something newer with better gear ratio for the purpose you plan. if swapping the diff simply get one with disc option is that's what you want. the other option is an overdrive behind the trans or an overdrive trans. here is a site that will give you engine rpm at highway speeds with your tire diameter, rear axle ratio and trans final drive gear ratio
https://www.randysworldwide.com/calculators/rpm/
another one for tire size comparison, with visual comparison as well as tire height charts
https://tiresize.com/comparison/
hope it helps.
on the front disc brake question, the kits with original hubs re-used will probably need the old drums removed and the new rotors installed in their place. the drums/rotors will likely mount to the rear of the hub with rivets/screws. those old hubs will likely be using ball bearings instead of the newer roller bearings. you may need to factor that in for cost and parts ordering if going that route. not sure but I think the kits that come with new hubs will install the rotor on the outboard side of the hub, like a newer vehicle. this makes it easier if doing brake work/maintenance. that said, it's not a bad idea to take the hub off and repack the bearings/replace the hub seals if doing a brake service. personally, I would look at the kits that use standard, OEM, brake components, from whatever vehicle, because the one off special order stuff sometimes gets updated over time with new designs etc. then the old parts become obsolete and unavailable. unless your plan is to build a trailer queen that never sees the pavement you will require a brake job at some point.