Hey Dudders,
In regards to your bad milege I had the same truck and I was getting about 11mpg. There was a point where it was pissing fuel and I was getting around 6 to 7. If you have accesss to a Snap On Modis use it. Check out the wave form on all of your 02 sensors and they should be going up and down from .1 to .9 V if you see one stuck high or not working or going up and down replace it. This would cause a really rich run and suck down your fuel. Another sensor that is over looking with chevy milege is your mass air flow sensor check that out as well. A Snap On modis you can check everything in about 2 minutes. Amissfire will hurt your milege too. Well let me know how it goes.