I was getting about 15mpg with a TH350 and 2:73 gears at about 75 mph. The car would go just under 200 miles on a tank of fuel. You will probably do much worse with the 3:73's. I guess the decision to either stick with the current setup or do some mods would depend on what type of driving you are planning. If you are primarily planning to do road trips, you will benefit from changing gears. If you are going to primarily attend local cruise-ins and the like, you may want to leave it alone. My suggestion is to take it on the road and see how it does, figure out what you want it to do, and go from there.