You need to find an SM420 transmission, which was the optional 4 speed for this truck. They are fairly common, and available for both closed and open driveline. They have a 6:1 1st gear ratio and weigh about 120 lbs. They bolt up to the stock bellhousing and driveline. The way to find one is to look for a 4 speed that has reverse to the left and back- that is, toward your knee and back. Avoid the ones with a lockout lever from '48 and earlier because they aren't synchronised.
