The SM420 would be a very easy conversion because it will bolt right up to the housing and factory floor pans are readily available to replace the three speed floor pan that your truck has.
There are two versions of the four speed floor pans for TF trucks. One was used on the 1/2 tons and just has the cut-out for the shifter. The other one was used on the 3/4 ton and larger trucks. It has the cut-out for the shifter and a second cut-out for the ebrake lever that was attached to the tranny on the larger trucks.
The 1/2 ton SM420's have a different rear yoke than the ones for the larger trucks that had the ebrake assembly.
So the easiest way to do a four speed for you, if you have still have the stock rear end, would be to find an SM420, drive shaft and floor pan out of a 55.2 to 59 1/2 ton SWB. Everything will bolt right in. 1960 to 1966 SWB four speed driveshaft would probably also work.
Matt
