I would take a grade 8 nut and place it over the stud. While holding it with a pair of pliers or vice grips, fill the center of the nut with weld. I have done it with my trusty mig many times . The heat will help too. Just did the exact same stud on a big block last week.
Yep, only way to pull a rotten stud out.
But the end of the bolt , grind a taper to a piont make a V to load a lot of weld in turn the mig feed right down so damn near tigging and heat up.
And if worried about the surrounding surfaces, a couple layers of masking tape or a 1mm layer of silcone sealant.
Dont even try putting a slot in the top.. the metal is rotten and will give far less to weld to WHEN that fails.
Dont go the helli coil route...avoid if at all possble.. those are for ppl who rip proverbial and bust...see below
Drilling/ Ezee outs....dont even bother if cant put the manifold in a drill press to drill square... And snap the ezee out inside u are in for even bigger problems drilling that hi tensile steel and If fails now have nothing to weld
If all fails, u can end up drilling out a std size larger, re tap the hole and there is a chance the carb will still drop over the slightly larger stud on that cnr.. If not its only a very minor drill out to make fit.
All of the above is 40 yrs hands on experience.. not google theory.
But what I would be doing is replace the 2 barrel carb with a modest duel plane valley cover and a 400 to 500 vac sec 4 barrel Holley.
Then maybe look at a modest street cam later.
Put a bit of life and better economy into the old girl.