I had a similar problem with a '54 **** I once owned. The problem there was that the transmission kept coming loose. When it came loose, it would sag in the middle (where engine/trans come together) and somehow cause the linkage or trans shaft itself to bind. If I jacked it up a little so the engine/trans came together it worked. You might want to make sure the bolts are the right size with good threads and tightened up good.

My father had a '50 GMC truck when I was learning to drive. It would not go into first gear if you stopped with motor running so you had to double clutch, rev to match engine/trans speeds and shift it into first gear while still moving at 5-10 mph. Never did figure out what was wrong with that. But, I sure learned how to downshift without a clutch on that truck!