an update.........
after cutting 2 1/2" holes, blending them together, and matching up the pipes, i ended up with a 1" x 4 1/2" window, top and bottom.
so i now get to fab two pieces to close those holes. the only good thing i can see about this goof is, i can get in there and smooth the internal mating surfaces some.
i hope i didn't screw this thing up.
an H pipe would have been MUCH easier to do !
all this work, and it will probably not make any difference.