Machinists, a little advice if you don’t mind.

Aluminum is very susceptible to galling .... we spend most of our time trying to prevent galling - but in this case may it may actually solve your problem.
 
I'll add another solution that's not been mentioned yet. I'm guessing the "stainless" receiver is aluminum. You could make a piece of 8-32 aluminum threaded rod and braze it in using an aluminum brazing rods. The brand that crappy sells are really great actually. You might have to turn that screw in to leave a hollow at the top to hold a puddle of brazing rod. Then drill and tap new holes in 6-48. If you layout the holes precisely and punch them, you could do the work on a drill press.
 
remove the helicoils and find/make/have made an insert with 8/32 STI threads on the outside and #6 on the inside.
or
remove helicoil and open to something more common than STI tap sizes, with a fine thread pitch, and bore out some stainless bolts for #6
 
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