[Elvis]

I probably have very little to contribute but in my view there are a couple of things you could check. My '67 has factory air so the set up may be different. There should be a water control valve on the passenger side near where the heater hose goes into the fire wall. If you have a vacuum problem this valve may not be opening up. That would not allow the hot water to reach the heater core. It is quite simple to take the bottom of the plenum off (at least on mine it is). You can then check on whether the heater core is getting hot. I think that if the core is a problem you will usually get a leak. Also, if you do have a vacuum problem it is possible that the correct doors in the plenum are not opening. I will watch to see if some of the real experts have any better ideas. E.