I agree that a dual-plane, vacuum secondary carb combination would probably work well for you in your application.
I had a 360 with a mild cam and installed a Holley 3310 (750cfm vac. secondary) on a Crosswind (Professional Products) manifold. It was a great combination with no "flat spots" and the secondary operation was so seamless, you couldn't even tell when the back barrels opened up. It ran low 13's with stock 340 exhaust manfolds.
The Edelbrock Air Gap is $249.00 at Summit; the Crosswind Chinese knockoff is about a hundred dollars less for a very similar design. Summit doesn't sell Professional Products, so if you want the Chinese manifold, you'll have to look elsewhere. I've had mine for 2 years+ and have had zero issues with it. The Crosswind has both L-A and Magnum bolt holes (plugs are provided for the unused holes), so one size fits all...
You pays your money and you takes your choice. When I bought mine, Edelbrock hadn't shipped any Magnum Air Gaps yet (my motor is a 360 Magnum), so, I bought what I could get at the time, and it's worked very well. I needed a manifold...
Hope this helps.
Bill