The bolt does not go all the way through and act as a carb mount. The carb mounts to separate studs.
The rest of that bolt could be taken out. I'm not sure if just welding a nut to it would work. It might need to be torched out. I used to take things that had rusty broken bolts to a tractor repair shop that had an old guy that could get anything out.
I have the ATP 101140 Manifold. You can see my review on Amazon. The casting was rough and I cleaned it up with a carbide rotary file. Basically porting it...but not really. I checked it for flatness and it was fine and installed with no leaks.
https://www.amazon.com/ATP-Automotiv...dp/B000IYCY3E/
You say yours was leaking between the intake and exhaust. Mine was leaking where the flapper shaft was sticking through. Probably could have come up with a way to put new bushings in the shaft.