you have a few options.....you could drill and tap to a size bigger....and if need be you can drill out the holes of the carb to match a larger bolt size..
you can JB weld as you mentioned and drill it....although I don't suggest it.
you could have someone who is good with welding aluminum weld the holes closed and then redrill to the proper size.
OR what I would do......I would drill the holes larger,tap them and run some heely coils in which would let you maintain the original bolt size that the holes in your carb are already sized for.
thats just a few options,hope that helps a bit