Cut the hole. Then Roll a piece of sheet metal over large PCV pipe Sticck it in from the bottom. Leave it long. Tack weld around the inside of the car with 1" welds. Go underneath and trim the excess. I welded mine from both sides completely so it could be metal finished. This is a B-Body but the same principle. Measure to the gas pedal first to leave room. The pice I used was small , thus the side seam. Works out great.
Doug