In addition to Jeff's post, keep in mind that to convert a 6cyl car to a V8 and add all the specific parts will be a big $$ and time expense which will most likely diminish any additional value placed on the car.
The only way I would consider restoring to original is if the body is in near perfect condition with no rust and there are many original parts that can be cleaned and re-used. If you have to replace only normal items like bumpers, door handles and interior trim, keep the car original. Otherwise, you might consider parting it out. I really don't think you're going to make anything off of it by making it a GT clone.
Another option is to do the bare minimum to get it running and driving, clean it up nice and try to make a couple grand off of it. Probably your best approach for most profit. Once you start doing a lot of work on the car and spending lots of time on it, your time investment will wash out most of the additional value added.
There really isn't much wiggle room to make anything on coupes, especially 6 cylinders. They're fun cars for some and I'm not knocking those that buy them, but they are entry level cars and just not very desirable.