PB blaster on the shift rails.
First guess is the rails are stuck in the top cover, sometimes it's necessary to pull it off and if you can get to them tap the end of the rails with a brass punch.

If when you have the top cover off the rest of the transmission looks like junk save the effort and find a replacement.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-