Stock exhaust sucks. Thinking about fenderwells for my early Dart. Looks like BIG holes need to be cut in the inner fenders. What about removing the entire inner fenders and installing 1 5/8" mild steel rollbar type tubes from the front of the subframe to the upper part of the firewall on a 1/8" steel plate welded to the firewall? Would this give the front end the strength and support that it needs or do I need to install a cage and tie it together?

Tie it to the cage.

Just cut the hole big enough for the tubes.