64 Dart Gt 4 speed won’t shift into gear ??

I am thinking that you may have damaged the clutch disc. After you take it apart, look closely at the disc, hub would look off. If you had the trans partially in and your arms were tired and you let the trans hang, you may have damaged the disc. Get 2 bolts about 2" long, same size as your trans to bell bolts and cut the heads off. Put these 2 bolts in the lower 2 holes as guides when you reinstall the trans. If and when your arms get tired, they will support the trans. Cut a slot across the cut off end with a hack saw or air cut off, if they come out hard you can use a screwdriver to back them out.
Good luck