I think what you really need for this project is a 3-71 as the outlet is a single opening and will be much more adaptable to a 4 bbl. manifold. The 4-71 has two discharge openings and would be more restricted by the 4 barrel type manifold. With the carburetor base of the manifold machined you need to fabricate and bolt on a 1/2" thick plate to support the blower and for sure you will want to weld on some supports from the plate to the manifold. I'm sure your boss's SBC manifold has added supports on it. You mention you have pulleys, what style? V-belt, Gilmer, serpentine? Regardless of what you have to drive the blower you also need to engineer a way to drive the water pumps. You might find some inspiration by visiting this website-
http://uncommonengineering.com/