You could use the four speed that was optional for your truck; basically a heavy duty three speed with a "granny gear" below the other three and a direct replacement for the threespeed transmission. These are pretty noisy transmissions. If you want to switch to open drive, you could look at the Ford 1984/1985 four speed (TOD T170FT) with fourth gear overdrive which had the shift lever mounted on the top of the transmission rather than on the tailshaft and would look original as well as the other transmissions noted.
