It seems there might be some money there, however turning a hobby into a business venture can take the fun out of it. I have seen a couple of businesses that build a specific car for a specific person (Coddington, Foose, etc.) The trick seem sto be finding the person with a desire to have a car done to their specs and with a fat wallet (I don't mean that in a bad way). Sticking with popular muscle cars does make the parts gathering easier. If you build a car to your specifications and hope you find a buyer it will make the task much harder. you also have the problem of finding cars with good sheet metal to start with.
I would also agree with considering the 67 to 72 Chevy short box trucks.
For your first attempt I would buy the best car you can to start with and see how it pans out. Starting out shooting for a smaller profit margin will give you an idea of how well your plan will work.