A bit of a suggestion. I was thinking fiberglass when you mentioned the smooth inside. Make the buck, paint it and smooth it with a urethane clear. Then go at it with some fiberglass cloth. If I was going to make re-pops I would either do it with the 'glass, or I would section the body, hood, fenders, valance, rear, etc. This would alleviate some of the molding issues. For some more insight have you tried largescalemodeler.com ? lots of guys there doing large scale stuff, including bodies