The 8HP90 that I picked up had a mystery converter in it. It didn't look stock and frankly I didn't trust it. So I called up Circle D and had a discussion about my requirements. Here is what they suggested:
This is their hybrid 8HP90 converter. Externally it is the same size as the 8HP70 converter, which is 1/2" smaller in diameter than a stock 90 converter, but internally it is built to work with the 8HP90 transmission.
Here you can also see the Domiworks bellhousing adapter.
and the Domiworks converter adapter.
Andrew