I've never heard of a heli coils hold in a cylinder head bolt hole not to say that they don't work. We wasn't the one's who installed it it was the machine shop, after the heli coil failure the block was junk. Are you using something other than a heli coil ?
I've never seen heli coils hold up when used in a stress area. I have a friend who had a block where the machine shop used heli coils in the cylinder head bolt holes, it never held a headgasket after that. The cap is a breeze to install, the problem is having the block align honed
The OP said that he was told that the engine was rebuilt before he got it. Myself, I don't see any other way but to pull the engine and fix it right. I don't believe in patch work or doing things half as*ed.
Dart Sport or Dart swinger? You should, that would be cool. I don't know if interior parts have improved in quality since 2007, that's when I had quality issues with Dart