we loved our 78
look for a support group on yahoo groups, like this one,
they aren't worth that much with no engine. the coupes even less. You want a fastback! What engine did it come with? check the vin, you don't want one that came with a 4 or 6, you would have to change the whole chassis, the v8 would be preferable, they are pretty easy to find, any 302 or 5.0 will fit.
I just sold a 78 King Cobra on ebay for $2900 and it was in good shape, v8 and all
It's one of the sleeper buys out there, Mustang II fastbacks for under $5k Ebay always has several.
But if you happen to have the right motor and are hooked on the car, offer the guy $500 he just might surprise you and take it.