I don't know - Thermostat isn't even close to 7 and 8 . There is No reason for that to happen.. ? Must be a coincidence is all I can figure.
There is nothing further back to leak . That heater tube is solid and 90's up behind the passenger side head . It's bolted to the back of the head . Your heater core hose connects right there between the head and firewall . It would have to be leaking their then. You should have a puddle under the truck if that's the case and your starter would have anti-freeze on it. You might want to get under the truck with a light and look up at it for evidence of it leaking their.
See the heater core hose back by the firewall - That's where it connects to pipe. The pipe isn't in this pic. -
See the water pump stem in this pic - the o-rings are removed, but that's what leaked in my case -