Got the itch . .. .

Took a huge road trip this last summer (1200 miles round trip) in the El Camino. Was really stoked to see what kind of mileage I could pull down. I was let down when I averaged just a hair below 13 MPG. It's getting me thinking that I should just forget it and put the 4.10s back in for the fun factor and say forget the mileage. However, $3.20-$3.40 gas in my area has me holding back. Back with the 4.10s the first round, I had a little 600 carb. Since then and putting the 2.73 in I switched to a 770 Holley, which I put a bigger squirter, pump cam and light secondary spring in. Now, it runs a lot better, but I'm afraid that going back to the 4.10s will get me like 5 MPG. I only put 2000 miles on it a year on average, but I think cutting my MPGs in half would severely discourage my driving it much. An OD trans. is out of the question, as we have a child on the way. I think I'm going to be lucky just to be able to hold on to the car. I wonder how long it would take an OD trans to pay for itself over straight 4.10s?