Looking at your third photo, there are three small projections at the rear of the top casting, the one with the shift handle in it. There is a shift rail and a lever arm that engages a gear ahead of each of them. Carefully remove the top of the transmission with the shifter in the "neutral" position, and you should be able to use penetrating oil and/or a little heat to get those rails freed up. Make sure all the gears slide freely while you're in there. On reassembly, make sure those lever arms engage the proper slots in the gears. The crash box is an extremely simple transmission to work on.
Jerry
