Best way to fix this 340 manifold

I think the best way to make the manifold functional is to get a machine shop to ream the hole and press a sleeve into the hole. You could even cover the sleeve with high heat silicone before installing. This method would seal the hole going into the exhaust chamber and give you a good solid hole for the bolt. Of course you could also install the bolt after filling the hole with silicone but some of the silicone might ooze into the exhaust flow space. You could also get a thin piece of metal (roof flashing) and using silicone bond it to the inside of the hole (hole in the exhaust flow chamber), bond it to the inside of the bolt hole. The small metal piece would temporarily cover the hole so you could pump silicone into the bolt hole without it leaking into the flow space. Then pump silicone into the hole before you install the manifold bolt plugging the exhaust leak.
But the sleeve would be the best method. You could probably do it yourself if you drilled the hole out just enough to get a sleeve (metal tube made of copper or steel). If you find a metal tube the correct size before you drill it out you can better pick the drill size. If you can get a good fit (slip fit) you could probably silicone the sleeve in place and permanently plug the hole.
GOOD LUCK>>>>>
treblig