Just saw someone do this last week...
Take the old clips with broken studs and grind the head down. The stud will come out through the threaded side. You will be left with a clip with a square hex in the middle.
Then buy carriage bolts with the right square hex. Grind the dome/head of the carriage bolt flat. You can put some JB weld on underside of bolt head to keep it from comming out of the square hex. But once tightened doesnt matter and not really room in there to fall out. All you are going to see from the inside of the header panel is the threads and bolts. And even those are completely buried.