There are still E body deals with 4130 full tube chassis.
I bought mine last year. Car came with 4130 chassis that will cert to 6.0. Strange struts, brakes, and rear koni's with a 4 link. Steel roof, quarters, doors, and the rest is fiberglass.

Unless you can build a new 4130 chassis for the existing body yourself, It doesn't make financial sense to have a shop do it.
If you want to go faster, I would sell your challenger as is, then get a fiberglass challenger body and have a 4130 chassis built for it.