1954 Chevrolet 1/2 ton trucks and 1954 GMC short-bed 1/2 ton trucks came with 3.90:1 torque-tube rear ends.
If you convert to the 54 torque-tube, keep your old 53 assembly. If you ever want to install the 3.55 conversion, you will need the 53 or earlier ring-gear carrier for use in the 54 rear end (or, simply convert the 53 rear end to 3.55).
Even with the 3.55 conversion, the 1st gear in the SM420 transmission will not get much use.
