With the Saginaw, most shift linkages available put the shifter too far back for truck use, which makes it interfere with the seat. There's really only a few choices on making one work, 1-cut a notch in the bench seat for clearance, 2 swap to bucket seats, 3 the custom bent shifter that red58 mentioned. There is a shifter from Patricks that clears the seat, but it is made for '47-'59s, I don't know if it will work with a '60-'66 body.
If you're looking for either a granny gear or a higher road speed, a 4 speed will not help. If you want the lower first gear, you'll want the SM420 4 speed, along with the different trans cover GrimGreaser mentions. If looking for better road speed, you should take a look at a T5 5 speed, which has an overdrive gear. That trans also has a provision for placing the shifter close to the same location as the SM420, so it will clear the seat. That conversion has been covered extensively, doing a search on this site will give you all the info you need.
