I don't think you will get much better than the 6-7 mpg you are anticipating. Also, I would leave the accelerator pump where it is. The discharge doesn't change much with the different positions. My '52 flat-bed would get 6 mpg up hill and down, 'fast' or slow, empty or loaded. Empty, it did a little better, but 'empty,' it still had an air compressor on the back that probably was about the full rated load for the truck. The compressor also had a 6-cyl engine. A good tail wind and long down-grades helped.
Harvester
