The issue with close driveshaft transmissions, is that the transmission oil lubricates the u-joint. Even if you can get a more modern grease lubed u-joint yoke on the transmission, you'd still have to find a way to seal the rear of the transmission where the yoke comes out. Given enough money and a capable machine shop, it would be possible to modify the transmission, but a switch to an open driveshaft style gearbox would be much easier. The open driveshaft is the easy part, modifying the closed driveshaft transmission is harder.
