[HO455]

Have you had the distributor dialed in on a distributor machine? Is your vacuum advance working correctly? Unless you have a real loss of compression issue I would look closely at the timing. Start with the ignition and get that dialed in and if no improvement is seen move to the camshaft timing. A retarded cam will never allow the engine to run efficiently.
Our commutes are very similar. Last year I went from a tired 305 with 3.07 gears in my Burban to a new 350 with 3.70 gears. The same T400 and carb before and after. I saw no change in fuel mileage. 12 mpg for the back and forth to work and 14.0 to 14.9 for straight highway. We did recurve the distributor for the new gears.