Alfa gtv for sure, I think that would actually make the most sense of just about any choice there is, they are very popular, and most of them (even the nicer ones) need tons of body work to keep from getting rusty again.
68-69 Dodge Charger
68-69 Roadrunner
65-66 Impala
68-72 Nova
78-81 Camaro
77-78 Trans Am
I have a preference for 68 and 69 Impala - Caprice with hideway headlights-but not enough of other public demand
Looking at this idea from a practical perspective, for it to work, there has to be a reasonable number of donor vehicles available.
It makes sense with Mustangs and the American muscle cars, but as the original car becomes more obscure, the potential market becomes much smaller. The 240z could probably happen, it would be a little harder for the Alfa (though they may have a strong enough following), but I doubt that you'll be seeing 510s or SAABs any time soon.
I think a 2 door 510 would have enough interest as well, idk about parts availability though.
Datsun 1200 would be cool.
There actually is a company in poland which makes mercedes w113 bodies.