65 barracuda driveshaft length / transmission question

As mentioned, you'll need to do something about shifting the transmission. Also, a later speedometer cable is needed.....66-early '68 model will work with your '65 speedo head. '66 works the best since it has the correct firewall grommet and is the correct length. As for driveshaft, I don't know what the correct length will be with your particular combo, but, once you get the drivetrain installed and weight on the rear end, you can measure what you need. Slide the yoke into the transmission, all the way. Pull it back out about 1 1/2". Measure center to center of where the u-joints will be. Assuming your yoke and pinion flange have the same u-joint size, (probably the 7260 2 1/8" joint) just get a driveshaft with the correct length if you're lucky. If you can't find one the correct length, get one a lot longer and have a driveshaft shop shorten and balance it. If you need to shorten one, get a driveshaft that has at least one end that isn't tapered. If you get one that needs to be cut right where the tube necks down, that won't work. Having one with the same size tube from end to end is the best type to use. It gives the driveshaft shop more options.