If you have a wrecking yard around with older trucks, look for 76-72 3/4Ton trucks. Check the ratio if you can, either by counting teeth or by spinning the wheel. If you get a 4.57 out of a later truck, you are half way there. On occasion, Randy's Ring and Pinion show a 4.10 gear set as available, but they can only go in the later pumpkins. Search Eaton HO-72 on their site.
When I mentioned swapping to another 8-bolt rear, I should have said 8-lug. I agree, a car rear doesn't belong under a 3/4ton truck.
Current fleet (subject to change w/o notice) \'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run. My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787. Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.
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