I've been completely out of pocket @ Philmont for the last 2 wks and just now got back to civilization.
That Stripped thread repair kit isn't the thing to ise on stripped metal inserts in a plastic tank.
I'd use a suitable plastic adhesive in the stripped out hole, press the metal insert back in place, apply a very thin fulm of grease/release on the male threads, thread the fastener in the insert.
Then give the fastener an eighth or quarter turn every 5 minutes until the adhesive hardens a bit.
No, really, the mustache means I love you.