saratoga54,
The 420 Ed referred to is an SM420, the optional 4-speed transmission from 1948-1954 - these years had a torque-tube rear end.
As Ed posted, the top gear on the 3-speed and the 4-speed are the same (1:1 ratio/drive).
The 3-speed and 4-speed manual enclosed driveshaft transmissions in those years are the same length.
Regarding the clutch housing and the need for a rear-transmission frame crossmember:
- the rear-engine frame crossmember changed in 1954
- the clutch housing also changed in 1954
- these changes allowed the elimination of a rear transmission cross member in 1954 (for 3-speed and 4-speed transmissions)
The 1953 rear end was 4.11
The 1954 rear end was 3.90 (lower revs at higher speeds)
Either/both rear-ends can be converted to 3.55 (check the Tech Tip - Engines section)
