So I let my truck warm up until it was at operating temp, took the cover off that supplies the vents on the heater box (right behind shifter) Set heater to vent, and there is hardly any air flowing around the heater core. But, I can touch my finger to fins on the heater core, and hold them there. It doesn't seem that hot. I have coolant flowing through the core, however, because both lines, in and out, are hot. I took the regulator completely out of the line going in as well. I am out of ideas.

 

At 400k miles you might consider swapping in a new 'upgrade' heater core from NAPA. The original is probly so full of gunk it's not very efficient.

 

im guessing u tried flushing the heater core out? at this point if u havent and you do, be careful, cause it may crack the core. sounds like bgilbert may be right
i did to mine awhile back, and at the same time put in a new thermostat. put in a 195 degree instead of a 180.
it nearly drove me out of the cab today. it had a bit to warm up, but its still really cold here and i took off all the jackets i had on till i was just in a t shirt, and i still had to turn the fan down to medium low.

 

The core is 2 years old. I flushed it less than a month ago. This is kindof an ongoing project because I only have weekends to work in the garage bc i'm at school during the week. However this project is taking way too long. I can't wait until it works right!

 

or for it to be nice and warm outside..... global warming my edit

 

exactly