You cut it out to the out side wall which leaves it almost square then where you left the bolt bosses you cut them down to almost nothing. Now you have room to round the runner passages and to polish everything up. Then you make your alum adapter that I mentioned Taper from the outside of the carb openings to the new manifold opening and install it with the longest bolts possible. I never depend on a plate to seal off the manifold I always braze the holes left in the bottom shut.
This will produce a street manifold that will handle a four barrel and have good torque. I may not make the all out horse power that a clifford will at high RPMS but if properly done you will be amazed. I tow my roadster behind my sedan at highway speeds with no problem. Many people on this board have seen it several hundred miles from home and some have followed it at speeds in excess of 70 mph while towing the roadster. I am not claiming this to be the super best you can get, but it does work well if done right.
Been there, Done that, Hope to live long enough to do it again.