First,
read the Shop Manual. The 3-sp and 4-sp transmissions are the same length but 3-sp and 4-sp u-joints are different. Just use the u-joint from the 3-sp.
You will have to use your 51's clutch-housing and you might as well use the 51's clutch assembly.
The 54 will not have a mount at the rear of the transmission but you should be able to use your 51's transmission mount on the rear of the 3-speed.
Tim
Oops, you will, of course, have to install all the 3-speed shift linkages, the 3-speed shifter box on the lower end of the steering column, and the 3-speed shifter arm and collar on the upper end of the steering column. Are there already holes in the upper end of the steering column for the shifter arm/collar?
