[ChrisHamilton]

Here's one that is similar to your description of yours.

http://forums.pelicanparts.com/porsche-cars-sale/1167004-78-911sc-targa-37-500-a.html

I don't know squat really but from your description I think $50K would be the very high side of what you could expect to get.

Check out some of the sales on Bring A Trailer. (bringatrailer.com) There have been quite a few 78 SC Targas sold there. That should give you a pretty good idea of what fair and what's not.