Carb too big?

yeah, I would call a Carter-brock a vacuum secondary either. Vacuum secondary used a venturi in the primary bore to actuate the secondary linkage via a vacuum pot. The faster the velocity through the primary venturi, the stronger the vacuum signal that was sent to the vacuum pot to open the secondary side, giving a progrsssive controlled opening. Carter-brocks used a weighted door to similar results so the motor pulled open the secondary doors that are on top of the mechanical secondaries. You can be WOT on a carter-brock and still be only on the primaries if the motor cant pull the secondary air doors open. Same on a Holley, you can be WOT on a Holley and not have enough velocity through the primaries to generate enough to open the secindaries via the vacuum pot. You can change springs on a holley to tailor the opening, you cant easily change that on a Carter-brock. The AVS had a better system as it used a spring door that had some adjustability built into it but still is not the best choice for a slant. Good news is you can buy venturis for a Carter that will neck down the primary side but the 750 is just too big for a NA slant. Sell it or trade it for a 500 or a 390. velocity is the key and to alter that you need to narrow the primaries for the other circuits to work properly.