Sorry, You asked about enclosed driveshafts, and my answer responded to that.
I should have been more specific: Yes, any 31xx series 1948-1954 truck 4-speed will be synchronized in up-shifts, and in down-shifts (except maybe from 2nd to 1st), AND, they should have the torque-tube cover on the rear of the transmission (easily confirmed by looking at the rear of the transmission).
The rear enclosed u-joint cover/housing on the 4-speed is swappable/changeable with an open drive-shaft cover/housing.
I do not know what improvement you are looking for: high end or low end. The high gear in both transmissions (and in the 318 3-speed) is 1:1.
