Do yourself a BIG favor and put a solid piece of metal under two of those carbs. That way you can impress the people who don't know any better, increase the gas mileage exponentially, and pull loads as well as (or better than) you can with an over-carbureted engine. The proper carb was a fairly large single 2 barrel with a rectangular footprint. Ed Pruss lives close to the Denver area, and he's for a good running 54 GMC 630 with a 503. It started out life as a fire truck. I'm pretty sure he can give you the straight scoop on what works on that engine. It's the same one I drove from middle Tennessee to Kansas City a few years ago after Ed had problems on his way home from where he bought it in Cullman Alabama.
Jerry
