The cheapest and quickest thing to do is scout your favorite truck wrecking yard for a suitably heavy axle assembly. If you're lucky it'll have the same U-joint flange. Also it would be nice to not have to move the spring saddles. You may even have some choice of Gear ratios. Forty years ago I got one out of a GMC450 and put it in a 57 350 but I had to buy a 12V shift motor. The 450's was 6 volt and spun way too fast. Once the dust settled it ran until the owner died about 10 years ago. He called once to see what oil he should put in when he changed it. He hauled lumber for his construction business. He worked it all that time.
