What's the best way to fix aluminum intake threaded hole for temp gauge sending unit?

Whatever you end up doing don't use a standard Helicoil. Having a round aluminum slug made, bored and tapped to the proper size would be a really good repair. The manifold would obviously have to be bored to insert and weld the slug, it would be a great repair. It is a shame how many aluminum bolt holes and coolant holes are damaged because installers don't go the extra step to properly coat the pieces before they are installed.