Christopher -
A Roto-Swirl CAN be used in that machine. The tub is a bit too deep really, but it can be done, especially if you don't use the extra-high setting. The tops of the agitators sit several inches lower inside the tubs of the large capacity machines, so water will raise higher on the Roto-Swirl than it would on a standard machine, but not too high.
It is extra important however to use the latest agitator cap if you transplant a Roto-swirl into that machine, even temporarily. The older caps (that can be removed by hand) are not designed to seal air under the base of the agitator as tightly as is necessary in 1978 and later machines. These have a seal that must be seated by a socket wrench to prevent someone from loosening it, which would flood the centerpost bearings and transmission with water.
Last time I checked, the new-style agitator caps for Roto-Swirl and Straight-Vane agitators were still available new either in white or gold. I would guess they're on ebay too. Do yourself a favor though and be sure to properly seal whatever agitator you put in there, if you get the machine.
I hope you get it and enjoy it!