The transmissions are different lengths, so different drive shafts. There is an exception, there was a heavy duty 3spd option. I've only seen one and that was in a 60 panel I had 30 years ago.
You will also need to shift it. A 3spd shifter on the floor, or find a donor and pull the column,all the linkage, and drive shaft. That's assuming you have the 4spd column now.
There are other problems depending on what year 3spd.
Larry
Last edited by LGriffin; 09/30/2009 9:42 PM.