No heat and I think I've tried everything...

[jonrbrt]

I have a 72 K10 Cheyenne Super with factory air. Always had good heat. Last summer I had to replace a broken defrost lever. Now I have no heat. Here's what I've tried.
1) Noticed water was hot on both sides of heat air control valve.
2) I suspected it could just be heat transfer and not really getting through--so I by passed the valve--still no heat.
3) Next I replaced thermostat with new 195. and while I had coolant drained, I also opened hoses to heater core and simply stuck a garden hose nozzle to one side to make sure water flowed through freely. It did. still no heat.
I get plenty of air through all vents defrost, AC or floor all levers seem to be funtioning correctly--plenty of air--just not hot.
what does inside air and outside air mean on top lever? Where does outside air get in... is it through the cowl?
With the truck in the garage and warmed up it will blow heat... but as soon as I start moving it gets ice cold. Even the box the heater core itself is in is ice cold. What can be going on?