I asked my friend who runs a few of them still. He said that yes the 478 only has the SAE #2 (round) housing.
So yes most any heavy duty transmission would fit.
The smallest Roadranger is the model RT(O)-610 or 6610 at 357 lb without oil or clutch housing, about 400 lb installed.
Eaton Fuller and Spicer all make some nice single countershaft 5,6, and 7 speeds that may not be to big. I don't know much about them, other than they are generally synchronized.
Grigg
