Good Frosty: The Monte Carlo frame is around 2 inches shorter in wheel base than the Mercs. I had to stretch one that much but I used most of the Merc floor. In your case I think I would hunt for a doner car with a matching wheelbase as the convert. I think some of the four doors are pretty close. Take a look at the late 70 mid 80's Olds Delta 88 and buick of the same size.
The photos of the ones that I have seen done use a lot of the firewall from the doner car so cut it high if you do it.
Frosty, there was a guy in the PNW who showed up at rod trots about 6 or 7 years ago with just such a roadster.. truck frame, dual rear wheels and a roadster body built from pieces. A monster sized rig that got a lot of attention.
Ink, there really arn't any set "rat rod frame" plans. It is pretty well "run what ya got and make it work". The whole concep revolves around taking somthing that is cheap or better yet free and making somthing hot out of it.