Ball and trunnion boot replacement

It is not the transmission that needs replacing. One is replacing the Ball and Trunion joint on the driveshaft with a ujoint (more modern). The driveshaft with the B/T joint unbolts from transmission. Take the whole thing to a diriveshaft shop and let the expert repplace it for you, it requires cutting welding and measurement to keep it the correct length. Then it needs rebalancing. In the future if you have a problem with the u joint you will be able to repair it easily. I had mine done for about 300 bucks, came out nice. The trans in my 65 cuda formula s is not original and is upgraded but that makes no difference it is still a 904 auto same size as original 65 unit. as the driveshaft was the original one!