250supersix - as to why the shorter adapter does not work as well, I can only speculate; but my guess would be turbulence.
Take a look at Tony's post right below yours.
When we were doing our testing, comparing a GOOD 1 barrel against the two barrel and adapter, we were unable to obtain the same fuel economy or power unless the adapter was shaped, and at least 6 inches tall. However, comparing the 2 barrel and adapter against a warped Rochester B, the 2 barrel and adapter would be better.
For the "do it yourselfer"; a good way to go is to fabricate a new manifold using aluminum with a plenum to match the two barrel of your choice. This can easily be done by using flat aluminum stock for the piece fitting the head, and various tubes and elbows of electrical conduit (from the electrical supply shop). Lay it out and heliarc weld the unit together. The carburetor flange can be made by acquiring one of the newer aluminum adapters, and cutting it to fit.
Of course, if a four barrel manifold can be found, a Carter 400 CFM AFB (9400s or 9410s) will work extremely well (albeit at more initial cost).
I put over 500,000 miles on a different vendors 6 cylinder (400,000 after installing a Carter 4 barrel). This was a 300 CID engine in an extended wheelbase van. At 70 MPH highway, the van averaged 22 MPG, and I have witnesses to this fact. Power was significantly improved over the original single barrel as well. I used an Offenhauser manifold for this project.
Jon.
