Using a chevy 350 as the op wants to how are you going to block off the intake to prevent fuel from entering and still get air in to compress with out building some exotic intake.
Bill
I would the KISS method. Why try to cut and weld on an aluminum manifold, unless you're skilled with TIG. I would make a sheet metal "plug" by bending some light guage steel to a shape that would fit about 2 or 3" inside the manifold, from the head side. I would bend it so you have a flat baffel, with tabs bent on 4 sides. I would then place it inside the manifold and drill through the top and bottom of that runner and run a sheet metal screw through each hole, into the baffle.
I'd then remove the baffle, and apply a liberal coating of JB Weld on the outer edges.
Once dried, I would then drill out an opening in the top of the runner, BETWEEN the baffle and the surface that contacts the head. Make it large enough to provide sufficient air flow (3/4" or larger) and thread it with a pipe thread to allow fittings to be attached. If you could, it would be better to totally open it, ie. cut it off from the baffle out. The more air in, the more efficient it will be.
Maybe some else has a better method.
Anothe thought might be that if you're using only one bank of the engine, have the manifold cut with a metal cutting bandsaw, to a straight flat, surface, that cuts off all the compressor side ports. Block them off with sheet steel and JB Weld (there's no pressure coming out the manifold) with the JB on the OUTSIDE of the manifold, and make new adapters to attach to the head and routed to your intake air filter.