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I can tell you the calipers from the larger rotor won't fit on the smaller spindle... For the hassle on the rear consider going to a disc brake set up.
Also the backing plates are different for JD5 vs JB3... .. and good luck finding them for JD5
You might try an adjustable prop valve in your line to your back brakes
You have a 1984 C10. JB1 and JB3 use ~1" thick rotors. JB5 and JD5 use 1.28" thick rotors. Both the 1" and 1.28" thick rotors are 11.86" diameter.