Andy,
I think I misunderstood you...I was thinking you just couldn't get it to go into 3rd or 4th. I reread you post again and I see your stuck in gear. Do you know what gear your stuck in?
With the clutch IN and the truck rolling, ALL of the gears and shafts associated with the gear your stuck in are turning. Did you try to pop it out of gear while being towed?
Jack the truck up (be sure to chock the front wheels) to be sure the stuck engine isn't acting like a parking brake, locking everything up with it's own weight.
When you had the cover off and you moved the 1st and 2nd shift forks did the rod the forks are attached to move back and forth or did the shift forks just slide on the rods? The reason I ask is some of the old transmissions had the shift rod move (forks are bolted to them) and they would extend out through the front of the transmission and into holes drilled in the bellhousing. Maybe the rod is rusted to the bell housing? Since I don't know 1946 transmissions I am just throwing it out there...
Mike B
