Not sure if you mean that you intend to leave the 76 cab wiring intact and connect it to the 1984 front/rear lighting and engine compartment circuits. Here are some dwgs that might help:
1984 fwd lighting/horns & engine compartment (with V8 -350):
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1976 fwd lighting/horns/ engine compartment (V8 -350):
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And the in-cab wiring for a 1976:
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Rear lighting is the same for both years:
LG = backup lights
Br = markers, parking & plates
DkG = RH directional & brake
Y = LH directional & brake
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And the pink lead in the rear harness goes to the gas tank level sender in both years too.
