I didn't think I had anything more to add, but I found another possible contributor to that nasty carbon buildup. It's obvious after today that since I've owned my truck the tstat has been stuck open. My temperature has never moved from about 50 or 60 C. It now definitely reaches operating temperature, and you can tell when the tstat opens (temp goes down 10 or so degrees).
I'm going to fill up tomorrow morning... looking forward to posting any increases in gas mileage

I've noticed a steady increase in mpg every tank since I replaced the CPI and did my lower gasket (to the tune of about 7%).
Sadly, I am still not blowing very hot air out of my vents still. I did an acid flush along with the tstat change and additionally I removed the hoses from my heater core and flushed a LOT of water through it. Any suggestions would be appreciative.