As the others have said, no. 78' & 79' swap between those two years and 80'-96' are interchangeable but you can't swap hardtops between 2nd generation (78/79) trucks and anything else. That radius of the front curve (right behind the doors where the body tub goes from vertical to horizontal) is different. The tops *almost* swap but not quite.
However, all this info is pretty useless if what you're asking is "will an 80-96 fastback softtop fit a 78/79 ?" and if *that's* the question then the answer is yes it should. Dimensionally the tops are almost identical it's the radius of that curve that really keeps them from interchanging and that's something that doesn't matter when dealing with a softtop. The dimensions you care about are the width of the body tub and the length (or distance) from the back of the steel roof to the tailgate and those numbers are very very close. I'm not going to say with absolute certainty that it can be done but I'd be very surprised if it couldn't. The top might be a bit tighter than it would be on an 80-96 but I don't think it'll be tight as a drum.
As an alternative the fastback softtop isn't a very complex design and you can easily have a local boat shop or auto upholstery place do it. I had one made that way for my first 78' and it cost me $200 from a guy who did boat covers. I had it made from a rubberized marine canvas and it worked great. It used a simple rail on the roof (the leading edge of the softtop bad a nylon rod 1/4"x4' sewn into it and that slid into the semi-circular rail to secure the front) and snaps for the other 3 sides. He added the snaps every 10" or so and I just installed them on the body to match. Having one of the full convertible style tops fabricated that way probably wouldn't be cost effective though. I know Jeff's just came out with the full-top variety for the 80-96's but it's more expensive than the fastback and truth be told I personally don't like the style, I prefer the fastback.
If you *are* interested in the full convertible top then you can ck out Jeff's & give them a call to ask if they think there's enough material there to make it work with a 78/79 which is dimensionally larger by just a little bit. The list the top as fitting 80-91. It'll fit 92-96 as well but has no provisions for the rear seatbelts or 3rd brake light so they don't 92-96 as an application. Might be that there's enough slack in the top to make it work with a 2nd generation trucks but Jeff's doesn't want to advertise it that way because some guys would be bound to complain about the tight fit. If nothing else it's worth picking up the phone to ask them about it.
http://broncograveyard.com/bronco/i-35601_...ted_windows.htm
Right now I have a 79' & an 88' in the yard so I just went out & measured the two tops for the sake of comparison & having actual numbers. I wish I had something to measure the radius of that primary curve with but I don't. Here are the other numbers. Taken at the very bottom rear of each truck, where the cap meets the tailgate, the 80'-96' cap is 72" while the 78'/79' is 70.5". The length of each cap was measured at roughly the halfway point between the top & bottom, the 80'-96' was 61" and the 78'/79' was 63". Measured diagonally from the top/front to the bottom/rear the 80'-96' cap is 74" and the 78'/79' is 75".