Hi Gary,
I put a GV OD in my 66 Belvedere. I had a new, larger radius tunnel rolled at a sheet metal shop with long "legs". I cut the original tunnel out as needed by using the new piece as a pattern. After welding in the new tunnel I cut the "legs" to match the floor contours. I made a filler panel to close off the gap at the front and welded that piece in. Finally I cut two 4" long slits in the original tunnel behind the new larger section and hammered them open to match the radius of the larger tunnel. Then I welded in two wedge shaped filler pieces to fill the spaces caused by hammering open the rear section of tunnel. It turned out pretty well and my carpet still covered the larger tunnel! BTW I am very pleased with the overdrive.
Mike LeFevre