Every ponycar ever built was a unibody. The closest you'll get to a frame is the front subframe on early Camaros (there may be others with a subframe.

The best solution for making a really good handling '70 Challenger is to buy a complete frame kit from Detroit Speed and Engineering or Art Morrison. They start at $10,000 and are supposedly a bolt-on with the latest suspension. You can even get IRS if you're not going to drag race it.

Those are only the first two that come to mind, pick up a copy of Street Rodder and look at the ads.