Dunno if there is a torque spec for extensions. When I put mine in, I made them 'safety tight'. In other words, I made them so tight I didn't feel they would come off, but not so tight as to strip or break anything.
Be sure to use some locktight on them (thread locking liquid) too, since this is a high vibration area.
Snug them down good, then check them in a couple days, making sure they stay tight.
I know that's not the official scientific answer you desired, but that's what I did 8 months ago, and mine are still there, and tight.
Good luck.