So, a Special side hinged seat from '78-'79 can be converted to fit the '78 Standard frame by cutting the pan and trimming the foam flaps. Reworking the Special latch will have to be addressed and the placement of the rubber bumpers possibly. A boon to have would be the chrome trim piece to help lay out the pan for cutting. You'll loose some of the pan teeth that hold the cover down on the sides.......pop rivet where needed.
I threw a seat together for me Bud using one of those aftermarket double saddle seat pans and a stepped foam and cover from a '78/'79 Special. The double saddle was for the Standard frame so no cutting was done there. The labias on the foam were trimmed and the cover was pop riveted on. Looks a sight better then that double saddle monstrosity........hope he likes it when he sees it one of these days (out of state storing).......lol