When I installed the MYA6 (GM 89mm) in my 51, I just moved the hole for the original SM420 shifter to the correct location for my 89mm. I cut out the tower and gas welded it into the right location and then welded in a patch piece into the stock location.
While I was at it I replaced the driver's side floorboard and make a bigger access hole for the master cylinder.
I had to rebend the shift lever to clear the bench seat, and I should have bent it a little more forward because it almost hits the seat when I am in overdrive.
I'll see if I can post some pictures if that would help you.
