Steve -
That machine uses the standard transmission/agitator shaft. This is a large capacity machine, but not the gargantuan machines made from the 60s to 1981, so a Roto-Swirl or Vari-Flex would fit, but they would simply be filled higher-up on a full water level (the Vari would probably look funny though). The large cap. agitators were designed to have water raise up higher on them as compared to the agitators in the standard tubs.
If you don't like the "cruel" action, which takes getting used to but probably works best of the belt-drive agitators for the larger tubs, the "winged" double-duty Surgilator would fit perfectly.
Happy Thanksgiving everyone!