That's because '63-'65 A bodies had driveshafts that used a front ball & trunnion joint, so the normal U joint center-to-center measurement doesn't apply there.
If by chance you've swapped in a later slip joint transmission, the measurement from a '66 Dart should get you in the ballpark; but the length will differ depending on what transmission you've got. but the length will differ depending on But there's still nothing like getting under there and measuring for yourself- especially if you've also swapped a different rear end under there, too. Use the method preferred by your driveshaft shop or supplier; and if you've got someone than can/will build you a B&T driveshaft, follow their directions PRECISELY- or better yet, get the car to them.
