It's listed by Edelbrock as a square bore, even though they have larger secondary throttle plates. I think their reasoning is that the carburetor will bolt onto a square bore manifold without requiring an adapter for throttle blade clearance.
So its just a square bolt pattern like the Holley . It is a spread bore venturi system though .
I have a 78 Dodge C-Class with a 440 and it has a holley 600 spreadbore QJ replacement carb on it . Runs as good as it can considering the comp ratio , cam and weight . Dont think a bigger carb will make a difference . Its a runner up to 3k..lol Then it just runs out of everything else.