Velocity stacks
Diablo
How are you arriving at your calculated 7" intake tract?
I assume these carburetors are the same size as the marine guys stuff. Try Bruce Hanson, Warehouse Hobbies, RC International/Cobra engines and I am sure there are others. I have a velocity stack installed on a Hanson modified Zenoah G23H in an XCell Gasser, I got it from Warehouse Hobbies.