The 4500 is a stout transmission... and from memory has .73 OD. With the aerodynamics of a '74 PW not sure the 5.2 will like pushing that truck in OD.

If tire size is original (?29" OD) may be OK with 4.10 rear. With big tires I think the 1.00 final drive in the NP 435 will work better.