3rd gear on the 3 speed and 4th gear on the 4 speed are the same,I just got a 53 GMC with a 4 speed,My 52 has a 3 speed,and I can't get to the brake peddle when it's in 2nd gear so I have to shift out of gear to stop I would quess I'd get used to it after driving it so I'll see.But I see no advantage to installing a 4 speed box.

Pete