The older Spicer differentials have different pinion offset (below ring gear centerline) and new gears don't fit the old housings. Not sure what year the change?
I had a 65 F350 and ran into this problem, I think it was 0.5" offset for the old one and 5/8" for modern ones. I was not able to find faster gears I wanted and swapped the entire axle.

•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
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-