'65 8-3/4 and 904 Trans, Driveshaft Length?

If the rear is in the car, why not just measure from the flange on the rear of the trans, to the center of the u-joint cup in the rear yoke? If not using the 65 trans with the flange, just put the slip yoke all the way into the trans, pull it out about 1 inch and measure from u-joint to u-joint centers. Doo this with the suspension loaded.