Heat sucks

[clemsonteg]

So the heat in my 70 C10 sucks. Always has. So far, I have replaced the thermostat, radiator, and all the hoses. Both hoses on the heater core are hot. I tried flushing the heater core with water and I didn't see any debris. I had the heater box off a few months ago and I cleaned out all of the leaves before I re-installed everything. I also checked that the blend door is opening and closing as it should.

I am sort of out of ideas. The only item I can think of would be a partially blocked heater core. Can anyone suggest flushing solutions/techniques other than water?