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For a street application I would use a vacuum secondary carb, especially for an automatic. A 600-650 cfm carb might be more appropriate for a stock engine, but 750 Holleys can be made to work very well on street 350's. Since we are talking only square bore carbs, another option is the Edelbrock unit (I run a #1406). What they may give up in terms of ultimate power production and fine tuning to the Holley, they make up for with ease of tuning and reliability.Those brackets are readily available at the local salvage yard. That's where I'd go to find them.
I second the Edelbrock 1406. That's what I am running now. Great carb for general reliability, driveability and gas mileage, and very easily tuned compared to the Q-Jets and Holleys I've dealt with. As for the brackets, I am also trying to find some of those very brackets. I'm visiting a few scrap yards this weekend myself because I can't seem to find them for sale either, neither online or at parts stores. I will post how it goes.