A dual range Hydra-Matic, although it is a 4 speed, is not an overdrive transmission and will not answer the OP's original request.
SM318 and Sm 319 were used with the V-6's but the parts manual has no reference to the Borg Warner Overdrive (R-10) transmissions. They were pretty light duty and COULD be added by you but do not appear to have come from the GMC Factory. You could look for a Spicer 2021 2 speed auxiliary trans. That could either be an overdrive or underdrive depending on your placement in the driveline.
I think the T-5 is probably the path of least resistance and the best bang for the buck. Of course a T-5 is not a 4 speed either. Perhaps the A833 od trans or nv 4500 may be other options if you can get one cheaply. The nv 4500's have skyrocketed in popularity and therefore price. But you will need to figure out the adapters for them.
