Sounds like you got a city truck. It probably has super deep axle gears for city use. Does the transmission lock up in high gear? If not, it's probably an AT545. If so, yes, likely an MT643. If you can get under the truck, there should be a data plate from Allison on the trans, with the model and serial number. (If Google can't decode it, any Allison shop should be able to.) There should also be a tag on the engine, with the engine number and power rating.
The good news is that the axles on these have dropout centersections, so a ratio swap is as close as the nearest big-truck junkyard. What ratio you want depends on tire size.
1993 International Genesis conversion
Briza...all orange cats share one brain cell
Valas...the tuxedo toothpick