Matt,

This type of product is way better than a helicoil:

http://www.repairengineering.com/keensert.html

I have used them to repair damaged threads in motorcycle engine castings, including spark plug threads. They do not unscrew like helicoils because they are locked in place by 4 keys. Other companies make very similar products. You are close to Industrial Pipe and Steel in South El Monte and they probably have a selection of inserts.

If you have access to a lathe it would be simple to make a stepped stud allowing you to simply use the next larger threads in the manifold and leave the carburetor flange unmolested..

Drilling the carburetor flange to a larger size seems like a really Mickey Mouse "repair". I sure wouldn't do it

Ray