I think the 76 Courier had disk brakes. I took its .5" longer and .040" larger wire diameter front coil springs and stuffed them in my 73 B1600. I had to modify the upper A arm shafts to allow for camber adjustment because the springs raised the front end over an inch and there wasn't enough camber adjustment to get the front wheels even close to zero degrees.
I know the 77 & 78 B1800 trucks had disk brakes. I doubt, in any case, that it would be a straight up bolt on conversion to a 74 B1600. The A arms, spindles, and springs will be different and probably require some mounting changes. Also the proportioning valve and Master Cylinder would have to be changed.
My 73 B1600 stops great, unless I make repeated hard stops (then the linings heat up and I get brake "fade") or heavy towing and loads are encountered.
Possibly your hydraulics are week or the brakes are out of adjustment. Each wheel has 2 wheel cylinders that are NOT auto-adjust. The fronts are single acting wheel cylinders and the rear are double acting wheel cylinders. If they are not leaking, I would check the proportioning valve piston to see if it is stuck. Also the Master Cylinder could be internally by-passing fluid. With good, clean, dry drums, good hydraulics, good brake shoe linings, and adjusted brakes the truck brake pedal should be almost at the top with a firm pedal and not much travel to stop the truck in a straight line just fine...
Hope this helps some!