These are 4 speed OD transmissions. Calling them 3 + od is confusing as there is a 3 + OD set up with the 3 on the tree and the electric od out back....

Anyway, there are several flavors of these.

The SROD has the internal shifter. There are also side shifted (much like the old performance top loader, on which these are patterned) and a top shifted version as well. First up on this list.

Domestic Truck Tranny Guide

2nd to 3rd is the grand canyon, otherwise, 1st-2nd is good, the 3rd to 4th great.