Thanks for the compliments & interest!
As I mentioned in the text of a previous post I'm well aware that the cardboard mock ups of the diffuser section are too big and I intend to cut them down before using them as patterns to fabricate the 1st actual diffuser to test. When I made them the car was 3' off the floor so I used some string to determine the maximum space that appeared to be available (after modifying anti sway bar mounting) and made the cardboard mock up. It certainly shouldn't be that big and would stall. I also made the cardboard versions longer than the actual diffuser will be so that makes them look even bigger.
When these cars were built they mounted the rear anti sway under the bottom of the shock/spring plates. So they probably created a lot of drag. The aftermarket suspension I had in the car previously had a bar that mounted to the floor pan of the body and the axle tubes so it was up higher than the stock bar set up.
The new GEN II suspension from Pro-touring F-body.com has an adjustable rear bar that mounts to the position of the factory mounts on the rear frame rails and under the shock/spring plate also like the factory bar. The location under the shock plates puts the bar in a position that would create a lot of drag without an under tray and limited the area available for a diffuser so I made some new mounts to attach the bar to the axle tubes. Once I can drive the car and test things I may need to modify the upper supports for the bar to change the angles.
Stock rear bar mounting position.
Bar flipped & mounted to axle tubes.