Seems to me 3/4 tons and larger already have larger rear brakes than front;adding disc brakes will increase the stopping power on the rear proportionally. It would probably be possible to make this set up work,but you'd have to consider how much line pressure you'd need to make rear discs work,how much pressure the drum rear brakes required,and how the difference would relate to the front brakes,then would your master cylinder provide enough pressure and volume to handle the change. You'd likely have to use a proportioning valve and I'd bet you'd have to replace the master cylinder. You might be able to connect and use the disc rear brakes directly VERY temporarily if you plan on converting the front to discs too but that would be very marginal and I wouldn't expect them to work right. Converting the front to discs might not be too hard;you'd need to build caliper brackets,make some rotors fit your hubs,and I'd SERIOUSLY consider whatever work it took to convert to tapered roller wheel bearings.
there's also caliper to wheel clearance to consider.You could very well end up changing to newer wheels or using spacers and longer wheel studs to clear the calipers.
OldSub is right-read everything you can about brakes before you start;brakes really are too important to just wing it.
Speed
