OK, I have a 59, but my problem was similar. Hopped up 350, 350 turbo, 3.9 ratio, 31 inch tires -- lousy mpg and way too much power for my suspension/steering/brakes (stock susp/brakes).
Long story short- I decided to sell truck and keep almost new engine and trans. I installed a used (very) 305/350 turbo from my sis-in-laws car. This is after I had changed rear to a camaro diff with 3.05 gears. The combination of 2 barrel, less cubes and gear ratio took my mpg from about 8 to 14 in town. I don't run it on the hiway (small town, no need)but I run the A/C most of the time. Truck was so enjoyable with the new combo that I have kept it.
700r4 is a good choice. Low first gear, overdrive. Combine that with the 373 you will get better mpg and still have decent power/accel.
Don't know if you can get the mileage you are looking for though.
