I don't see why you cannot use a one piece driveshaft. The SM420 is much shorter than a TH400. When I replaced my torque tube system with a new rear end and a newer SM420 (last one GM made; has backup light switch), I wanted to use a one piece driveshaft. My 1980 long box pickup with TH400 had a one piece driveshaft and my 2010 pickup has a one piece driveshaft, but the driveshaft for the 1954 would be longer due to the short transmission. I spent a few hours looking around the local wrecking yard trying to find a one piece driveshaft that was long enough without any luck. Finally had to settle for a two piece driveshaft with steady bearing out of a long box truck with manual 4 speed. My local transmission shop cut the shaft down and balanced it for me. I asked the pro at the shop about using a one piece and he said the length would be too long. I should have talked to him before going to the wrecking yard.
