Is a 1978 Mustang II a rare?
No, the mustang 2's are the black sheep of the brand, they were good enough to keep the brand going, but basically being pinto clones they are not very well liked by most mustang fans/owners (plus they were pretty cheaply made)
I don't know if they'll ever be classics, (sorta doubt it)
The Problem w/ 70's stangs is the HP output was crushed by enviornmental constraints during the decade. Starting around 71 the HP went down every year, for example a 72 Mach 1 w/ a 351C only put out about 170HP (275 HP @ best)
However early 70's stangs are just starting to become collectable/desirable, whereas a few years ago they weren't. So really only time will tell if they'll ever be worth it to anyone. But as for the immediate future, don't count on it.
If you're looking at it to buy I would either go 73- or 79+ if you want anything of value