The least expensive would be to locate a FREE 3:55 ratio car rear end say, 1951-54. These were in cars with the powerglide trans. I have a tech tip on this you can check out.
The very best way is to ditch the torque tube and go open drive line and then a zillion better ratio rear ends could be used.


It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed