Give me a shout i will look at it for you, buy me a beer.
Helicoil and timeserts are two completely different animals. the helicoil is just steel threads, they don’t have the same heat transfer ability as aluminum so its not uncommon to have to go one heat range colder on that plug.
There is no way to insure the helicoil goes in 100% straight, sometimes they are off a hair which on a regular bolt makes little difference but on a plug it can cause sealing issues and ultimately failure.
Inserts on the other hand are full thread inside and out, they are anodized high grade aluminum and use a centering tool when reaming, drilling and tapping so they go in straight.
They are countersunk, locked in place and expand and contract at the same rate as the head, the heat transfer is the same so the plug heat range is unaffected.
That being said on an iron head engine you can probably get away with a helicoil if you get it straight, it still isn’t a repair i would do.
Any job done properly is a good repair that will last as long as the original or longer but when you start half arsing things it usually end up causing problems down the road.