I have the 1962 C10 with the SM420 (4 speed) and 3.90:1 gears in the back. It doesn't matter if you have 3 or 4 speeds, these trucks are limited to what the rear end gears will let them do. Both the 3 and 4 speeds from this era are top geared at 1:1. A more modern tranny might give you an over drive, but the way to increase top speed is the rear end gears.
If you are concerned with getting rid of the column shift, then the SM420 will do that, but then you need to change the tranny hump. There were 2 styles, high and low hump. Auto (powerglide) and 3 speed used the low hump and are the most common. The 4speed used the high hump, while not rare, they are less common. It's not a straight bolt on either. You will need to replace the removable tranny cover bolted to the floor of the cab, but you will also need to replace a piece that is spot welded to the firewall. Finding those is getting a bit tricky, but they are still out there.
