small block 340 driveline in a big block 727

Trying to get our driveline installed on our 73 duster, but for some reason it won't go in. So my question is this: the driveline is from a 1970 plymouth duster 340 and the transmission is out of a 1970 plymouth sport fury 383 car. Are the splines the same from one to the other? Could the transmission rear seal be different? Or am I just not getting it lined up right?