Assuming you are 4WD, the "easiest" way if you wanted to use the '91 speedo cluster, would be to swap the transfercase for an NP241 from '90-91.
'90-91 used electric speedometers, you need a VSS on the output shaft of the transfercase if 4wd.
Anything up to '89 will work otherwise, remembering that somewhere in the late 70's all the gauges used electric senders, which isn't a problem, just need those from the donor. You'll need to re-pin, and probably add wires to the instrument panel connector in any case. Not hard, just takes a bit of time and thought.
Edit: And since you have a donor truck, even if you thought about swapping the NP241 from the donor, you'd need to do something about the transmission, because your 465 (assuming that's what the '73 is) is a 10 spline output, the NP241 is 32 spline input.