Think with the proper size helicoil you may have to pull the manifold due to the exhaust manifold hole diameter being smaller than the new pilot hole in the head needed to insert the helicoil ? I had a exhaust manifold bolt break two weeks after a cylinder head install. I had to drill through the old bolt using a 90 degree drill attachment and retap the hole for a slightly larger non-metric exhaust stud I got at the hardware store. Worked well for me. Don't know if you can retap the hole in the head, (its soft aluminum) for a slightly larger size stud as you think you had a 3/8 thread in there already. May want to verify what you actually used ? I used a 5/16-24 tap, but may not work if you now have a hole larger than 3/8?