Firstly the "main shaft" is the one which exits to the rear with the propshaft flange on. (or in your case via the overdrive)
The shaft at the the front end is the input or first motion shaft. I think you cannot turn this one ?
The input shaft always is connected to the laygear, so it may be something obstructing the laygear rather that the input shaft.
If you have had the top plate off, then possibly the reverse gear may be in the wrong place.
suggest take top plate off again, ensure all gears are in the neutral position, & take a very close look at the l