I have actually seen a cage built like this. A buddy of mine was lookin at a supercharged mustang, the owner also owned an exhaust shop.
We went and looked at it and I told him not to buy it. It had a 2.5" exhaust pipe cage and the front bars and rear bars were not even welded to the floor. Not that it would matter, but the hoop was welded to the floor and the rear bars went into a speaker opening and the front bars you could wiggle back and forth.
Well he bought it anyway and we cut the cage out immediately. It was too good a deal to pass up. The guy had about $20,000 into it and sold to it to my buddy for $6400.
After it was stolen we swapped the drivetrain into a coupe, straightened the body out and painted it, then he sold it to another friend. That car is fast.
Low 10 second car.
But please never never do this. (exhaust rollbar)