There's qjet guys and then there Holley guys because you can take them out of the box and they work pretty good. Sounds like your mechanic is of the later persuasion. 750 is not too big for a 400. A few years later they were installing 800s on them. I ran an 850cfm Holley on my 350 at one time. It ran like a raped ape. Truth is your motor pretty much decides how much fuel and air it can use regardless of carb size. If you want your car to run right with the carb you have you will have to find someone that knows how to tune your carb properly. If that's not an option buy a Holley 750 and be happy with how it works out of the box. Or better yet spend $2K on a new fuel injection setup and never think about it again.
