If the air injector tubes are held in by gland nuts like the small block V-8's were, just heat the nuts with a torch to break the rust loose, unscrew the nuts and remove the tubes. Then thread the holes for 1/4" pipe plugs. I've done hundreds of them that way. It's a quick, one-time fix that will last the life of the manifold. It only takes about 2 turns of the pipe tap to make the thread fit a plug. If you use Allen-socket pipe plugs, they don't stick up above the surface of the manifold.
Jerry
