would you be looking for the Steadfast Manufacturing sub rail and floor system that moves an A floor over the 32 hump on the rear part of the frame? they sell on ebay.
http://www.welderseries.com/blog/online-store/rear-crossmember-model-a/ http://hotshoehotrods.com/chassis-parts.html There are a few others that I can't think of right now. I think speedway has one . http://www.speedwaymotors.com/Model-A-Rear-Crossmember-U-Weld,62095.html