302 head exhaust port bulge and a '65 Valiant LH manifold

You're going to WAY too much trouble. Here is what I do. I tap the air injection holes to 1/4 20 and get 8 1/4 set screws, coat them with red high temp RTV and screw them in. Over and done. Works real good. "Sometimes" you need to drill them to the correct size to tap to 1/4 but a lot of times you can tap them as is.