Rick,
I thought a SM-420 was a MUNCIE, not a Saginaw.
Something is rotten in Silverton. If this transmission worked before you did the rebuild on your truck, it should work now. Can you shift through the four forward gears and reverse? Can you actually feel those positions and their detents.
Could someone have removed the tower and reinstalled it with a shifter fork off the slider? Did someone use the shifter as a lift point when removing or reinstalling the transmission, and maybe pulled it out of the tower. Something has happened. Short of an internal gear freezing up, this is a unusual problem.
Do some more detective work, shift the lever around and let us know what you find.
Stuart
