If you want to permanently plug it all the way to the exhaust port bowl, you have to take the heads apart and clean them, place them on a steel bench intake face down and clamp them tight to the bench. Then you have to get some sort of small iron pot to put chunks of scrap aluminum in. Then you get a big-*** torch, heat up the aluminum until it's molten and then pick it up with big channel locks and pour it into the exhaust port from the bowl side. Once it all cools, you grind the aluminum in the bowl until it matches the contour of the iron. If your valve job, etc is ok and you didn't nick a seat, put it all back together and you're good. If not, do whatever other head work you need.
I've done this numerous times. Works great, but also, I'm in California.
On the 340 in my old Barracuda, I just made some sheet metal pieces the same thickness as the intake gasket and glued them in with silicone.
