I was very lucky with a propshaft company who balanced my TR3A propshaft which was about 8 years old and low mileage. I arranged for my son-in-law to help by bolting the front from inside while I lay on my back underneath holding the shaft in the right position.
I used new bolts, 107960, from Moss and their special locking nuts, ,GHF243 Cleveloc, I expected it to be easy but the new locknuts were a very tight fit and with limited room under the car it was a very hard push to move the nut and it took many small movements to make it tight. To complete all four bolts took nearly 25 minutes