By far the easiest and most cost effective method is to rebuild the stock system adding PS/PB to the existing axle. There are lots of kits available to use. In order to achieve your 75mph goal a engine upgrade is probably in your future along with a rear end swap.
The other option and a bit more labor intensive is a frame swap with a modern 3/4 ton. There are challenges with the swap to make it look right but you see it at the Rod Meets all the time, so it can be done. Good luck.

Martin
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