A willys cj pto comes off the transfer case, so at the PTO shift unit with the tranny in 3rd gear (direct) the shaft turns the same rpm as the engine. I am not sure what the gear reduction is on the bumper mounted pto box. You will need to find out what the 715 pto turns at, if it is the same as the engine, then the Willys box would be the way to go, if it turns at 1/2 the engine speed then you would need a lot of rpm to get 530 at the back end. If you need a Willys rear box I have a spare I would sell. Interesting project!.
Lee