***Somebody with a 68 confirm that I'm not talking out of my @ss!***
On my 71, the vacuum switch (pic 1) and temp sender (pic 2) are on the opposite sides of the manifold...not that I think it matters. On the vacuum switch (pic 1), it shouldn't matter if the hoses are switched. Try bypassing the vacuum switch and see if it makes a difference.
But then again, I've been known to be wrong...a LOT!
C.J.