I actually have the hardcopy catalog but I don't think it has all of Summit's inventory. Here is the link to Summit's site:
www.summitracing.com/ Type in 'carb spacers' and 'carb adapters' in the search function.
However, I think I found what you want when I searched the Summit site: 2 adapters made by Edelbrock. The Summit No. is Edl-2692 @ $32.97, and Edl-2693 made of aluminum @ $39.97. Both are spread-bore carb to square-bore manifold adapters.
Summit's toll free number is 1-800-230-3030.
Hope this helps. If you get one let us know how it works.
Rusty