The '78 pipes fit differently in two ways (from pre-77).
The way they attach to the head is different. This one is easy to resolve. 77 and 78s have studs at the exhaust port. A flower with 2 holes slides over the studs (with inserts pressing the pipe in). pre-77 has a piece (flange?!) with 2 screws holding it to the head - the pipe slides over the flange and gets "clamped" to it. The head is the same.
The other way they're different is way they attach in the back. 77/78 pipes seem to sweep upward more and the footpegs in the back of a 77 and 78 are in different spots (77/78s are higher). The back of the pipes attach to the footpeg. Earlier ones, directly, later ones with a bracket.
It seems hopeless for you so the 78 pipes should go to me, right?
I really need a set.
If you're going aftermarket, the bracketing issues are less of a problem. I may be doing the ones from Cyclexchange (black 4-2-1).