Yes, the intake (common chamber) has to come off to replace the thermostat.
The replacement I got from Merlin came with a new O-ring. (the only gasket in this app).
If you do go in after it, be aware of the hard (metal) water pipe that connects to the back side of the thermo housing. You have to unbolt it from the block & carefully wiggle it back out of the housing.
It is sealed by an O-ring that's easy to nick, & you will never know until it's all back together & you're adding coolant...it's a real pisser...
I repaired my FUBAR with hi temp RTV & it's still holding after 2+ years, but I also have some spare O rings from Merlin on board...if I can find them...
Also, if you end up pulling the intake plenum, you might just as well replace the intake gaskets & make sure to torque them to 13 ft. lb., might wanna check the shop manual for torquing sequence...

Good luck with this David, you seem to have had more than your share lately...