It's all about what you can afford to pay. I'd love to do a frame off on the '76 - I just don't have the knowledge. I know I wouldn't put $60K into a custom car, let alone an all original less they're going for that amount or more.
Realistically though, you ought to be able to blast the frame/underside for what, a grand, if it's in decent shape. Problem is you don't know what you'll get into so it invariably costs you more than you thought. Paint/body, wha, 5K to 7K. Engine is what, 2K to 15K ... it's all what you want to do with it.
I can't comprehend what is in these $60K/$70K numbers ... it's got to be all new parts and top of the line labor and workmanship.