Ted is correct, the final drive ratio is a combination of the pulley ratio and the worm gearbox on the driver axle.
I made a poor man's lathe by pressing the 18-tooth pulley on the shaft of a spare Pittman motor and used a sharp chisel to remove most of the outer "belt keeping" flange from the pulley. It then fit into the frame without needing to modify the frame itself. I don't recall needing to adjust the motor position for this project. There were others that needed it, but I don't think this one did. To be honest, most of these mods were done 10 or 12 years ago so memory is a little foggy.
