Possibly OD but uncommon, tag/model info will help or make of transmission and shift pattern can help determine if OD or not.
Some might bolt in place of an SM420... except from a medium duty truck probably has longer larger input shaft. Larger is OK, you can get a clutch disc to work. Longer is troublesome, easiest is an adapter which might be handy anyhow to adapt likely different pilot diameter in bellhousing, and could possibly deal with adapting bolt pattern if an issue.
It may or may not be easy but likely possible to swap transmissions either way depending on how resourceful you are.
First find out what transmission and if worth swapping.
