That may answer the question I have had regarding the different design of the runners of IDA Manifolds aside from how the carbs themselves mounted on the manifold. Some used on the GT 40's had a "bulge" immediately under the Carb mounting face that created a small plenum(?).
Example of each:
1.
http://cgi.ebay.com/ebaymotors/Cobra...spagenameZWDVW
2. (RRRGHHH) cannot find the other, it was on Ebay last night. Maybe it was pulled