Unfortunately, the tires aren't going to make a significant difference. The mods will help, but 7000 lbs is just a lot of weight for the H2. You were probably stuck in first gear at 37mph. I just pulled 3500lbs from Portland to Yuma , and averaged 54Mph over 1280 miles; avg 7.4 mpg. Ouch! I run the flowmaster and 35 inch tires. (these numbers are confidential and should not be shared with any trolls from fuh2.)
Take the radiator cap off and drive a new Duramax under it and put the cap back on. That will fix the problem.
