Check the sticky disc brake thread, you may accomplish the same goal with less work by just swapping axle components and rear axle to the old frame.
But an 80 might be a little big for nominal 1.25 ton pickup axles?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-