Need Heater Help

[lts70]

I have installed a Sure-fit system from Vintage Air in my 1970 Chevy C-10. I have everything finished and I am very impressed with the system and how cool the A/C gets. My only problem is that the heat is not working for me. I installed the custom coil unit so that I could shave my fire wall and run a 4-way bulkhead. I have the heater valve in the cab in-between the unit and the bulkhead at the firewall, on the line coming from the intake manifold. I took the hose off last night at the valve to see if for some reason the valve was not letting coolant through. When I had the hose off it drained enough coolant to fill a 44 oz. soda pop cup. I put it back together and ran my heater all the way to work this morning (20 miles, 30 minutes) it blew cold air the entire time. I put my hand on the valve itself and it was hot so I know fluid is getting past the valve and that it is hot enough. Is there something I am missing? Can my heater core not be letting the coolant through and no circulation is my problem? The only thing I can figure is that some how the heater core is blocked, I thought it might be some sort of vapor lock from air in the core maybe but after taking off the hose and having the coolant come out I ruled that one out. Any advice on how to solve this would be greatly appreciated as winter is coming and this is my daily driver right now.