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There are a couple forums that might be able to help you. Try looking at the Electrical systems/wiring, under Miscellaneous and your vehicle year groups(s) under Older/Classic and antique trucks section. Both can be found by clicking on the Forums block in the upper left hand corner of your screen.
Didn't know about those sections either, and I've been here long enough I should have browsed further!
And no, the two harnesses are not the same, but yes, I would think your '79 harness would easily retrofit to the '73. The frames are the least "wiry" of the sections of truck and most of the business is done in the cab and engine compartment.
So in theory, it should be a relatively easy swap with a few custom tweaks.
I do not know about the overall swap working out, but I do know that both trucks will need to be instrument panel matched with either all gauges or warning lights. The is a difference in the wiring for that much from what I understand.
Meaning if the 73 truck has all instrument gauges (fuel level, temp, oil pressure and Alt) and the 79 has warning lights (for those 2, OP & ALT) then the warning lights cluster will need to be moved to follow the 79 harness. Make sense?
And along those lines, the alternator regulator wiring is swapped around a bit between the two cluster styles. So there are under-hood changes as well.
But nothing that can't be done or undone, and you're most likely going to use the cluster that came with the truck anyway, or you may be in for some serious re-wiring behind the dash.