A serious vacuum leak can (and will) build heat, simply because it's an uncontrollable lean condition. I didn't pay a lot of attention to what heads/intake is being run but one has to be careful anymore, due to the many varied head/intakle combos as well as port placement on the head. In some cases, only one gasket will fit and align correctly.
You also should check for misalignment of the head ports/intake runners. Some heads will only take an intake made specifically for them. Been there done that, too!
And, some gaskets are subject to "Edge soaking" and then leak. We coat just the edges (insides and out) of our intake gaskets with POR15, and seem to have negated this phenomenom.
Another contributor to the heating, if it is a vacuum leak, would be the oil consumption. Oil burns considerably hotter in the combustion chamber than does either gasoline or alcohol, and if your intake is leaking, chances are it's pulling air/oil vapor out of the cam galley. If that's the case, another giveaway (besides the inability to idle smoothly or at all) will be a fine coating of oil and burnt oil smudge inside the headers . . The oil vapor burns so completely that it's nearly impossible to see in daylight, so it's something that most people don't notice . .
On the "Hardblok" . . We ran a 468, and a 496 BBC, both with filled blocks, and then went to a 582 with open water jackets (Dart) and the temp has been the same. 160° out of the water, about 170° at the top light and 185° at the other end. Leave the pump and fan run while towing back and it'll be about 100° at the thermostat housing when we get to the trailer . . .
Just some things that crossed my mind on the subject.