The 140 seat frame has four studs on it, and an adjustable track so you can slide the seat back. The studs go into the holes on the Super A seat supports. I guess a Cub seat frame would work but it would definately take some do-ing. I wonder if you could drill 4 holes in the seat frame, the width of the seat brackets on the Super A, and bolt it on that way?
If you can get it to work, you will like it. My granddaddy's '48 Super A had one added to it in '61. When I restored it I wanted it "factory" so I put a seat pan on it. I cleaned the old deluxe seat up and put it on Old Ugly, the Super A I got from Billy Clay. Lots of Super A's around here got retrofitted with the deluxe seat, and you really have to look to find anything newer than a Farmall 100 that didn't come with a deluxe seat from the factory.