If the trans has the 10 spline output shaft, it is not correct for a 65 GS. !0 spline input is correct. As someone mentioned, it could have been swapped in place of the original shaft when there was a failure in the trans. The 10 spline shaft is an inferior design. The splines are deper and it takes more meat out of the shaft when they are cut. As a result, most all of the 10 spline shafts have a twist in them from torque over the years. The twist starts right at the end of where the slip yoke stops. What then happens is when you hit a bump, the suspension compresses, and the driveshaft slides forward, and the yoke binds on the twisted splines. Causes vibration problems and causes premature failure of the u-joints.
Take some good pics of the trans when you are out there and post them here, We can help you identify what it is.