'73 Disc conversion, but keep 4" bolt pattern

The small bolt pattern is/was 7/16 studs and left hand threads on the left side. One could go to a whole lot of effort and expense to retain that setup using the later model factory OEM spindles. Those who do use the later model factory disc brake assemblies end up changing all 4 corners to 1/2 right hand studs and 4 different wheels, all to retain some level of uniformity.
Much more simple, typical solutions are available. Those who want disc brakes and retain the OEM small pattern setup and whatever wheels they have usually opt for either OEM K-H disc brakes or aftermarket disc brakes, like Scarebird for example. Try this analogy on your dad... The kitchen needs painted. You have 4 gallons of paint on hand but its flat black. That might help him take the most practical path to his goals.