Did they make 5x4ā€ axles for 8 3/4 rear ends?

Who cares what year the axle came from. Early cars had 8 3/4 with SBP. If that is the pattern on the car, that is what he needs.
If you order axles from any of the manufacturers you spec what you need. If you have C clip axle retention and intend to race, many sanctioning bodies require the big Torino ends and pressed on outer bearings and retainer. They make the axle to fit the bearing for any housing and width. Splines are speced also.
Iā€™m not racing it. I have a built 727 and my rear is Posi with 3:73 gears. It should be about the same as the new Challengers.