Maybe this thread should be moved to the Driveline Forum?
Agreed and done.
Garageguy, to answer your questions. Yes, the transmissions and bellhousings will swap in without issue. The 4 speed and 3 speed torque tubes should be the same, the only difference would be the front yoke and bell on the rear of the 4 speed vs the 3 speed.
Gear ratios for the SM420
1st - 7.05:1
2nd - 3.58:1
3rd - 1.71:1
4th - 1.00:1
Rev - 7.05:1
Gear ratios for the 3-speed. (from 1951 Service Manual - apparently the later 53-54 heavy duty 3-speeds are slightly different)
1st - 2.94:1
2nd - 1.68:1
3rd - 1.00:1
Rev - 2.94:1
So it looks like your starting gear (2nd on the 4 speed,) would be a little lower than on the 3-speed, just meaning you should shift out of 2nd a little quicker than you would our of first with the 3-speed.
Reverse is also way lower on the 4-speed (so NO RACING in reverse
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