Joe, I have done about the same thing you are attempting. I used the orig. cross member and notched it as you suggested. I used the original rear taans mount from my 3spd and welded that into the notch. Then I used a standard T5 rear rubber mount and bolted that to the orig trans mount.
Check out the tech tips, there are some good pictures of what I'm talking about. If you can't find them let me know, I think I still have them on my computer.
Good Luck
George
