The 79 block doesn't screw on...the 79 tube is not threaded, the block slides in and is held in place by two side screws, so its not going to be a part you can interchange with a 78's end cap.
Is a way to kind of do what you're thinking about.
Above is a single 12gr. adapter...which isn't real expensive..but with 1/2 the co2 on tap, don't get a whole lot of shots.
So with some adapters, could run that gun from a 20oz. bottle on a remote line, a 9oz. co2 tank, a 13ci HPA air tank, or 12gr.