I believe that the 8L90 top of the bell housing bolt was moved (to accommodate the direct injection), comparing it to the 6L80.
The 6L80 bell housing is its a full circle all the way around and not notched like earlier GM transmissions (e.g. TH350 / TH400 / etc.) are for torque converter access.
E.g. 4L80 on the right, 6L80 on the left:
I'd talk with Speartech:
http://www.speartech.com/
They do harnesses, etc.