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

Well, It's a possibility. Your going to have to take the trans back out, and take a close look at your clutch assy, remove it and look for the issue. Oh, when you installed your Trans, did your arms get tired, you stopped to rest, did you leave the trans half stuck in? If so, the weight of the trans was probably hanging on the clutch disc. That is a guess on my part, but you have to start looking.