I too would use the 3.23 in your case . Different engine builds are most efficient at different rpms, therefore, it is possible to get better mileage with numerically higher gears in some cases, however, IF you have an auto with a stock torque converter and fairly wide tires, the cam you have is pretty small which is why you can roast the "tire" with your current 2.73 gears, which also suggests that going to 3.55 will further reduce your mileage.
Another test to do is floor the car in first gear until it stops revving and tell us where it stops . If it stops at around 5200, then in my opinion, you will get worse mileage with 3.55 gears and probably even with 3.23 gears and that 3.00 would be the highest you would want to go.
Going from 2.73 to 3.23 will definitely be very noticeable but is not a massive change . Going to 3.23 will be sightly noticeable but not a day and night difference.
As mentioned, you should tune your engine, and a big part of that is setting your timing "curve" . This involves more than just setting the timing for the highest/best idle, and if you know how to use a timing light and have a good ear or tach or vacuum gauge, I can tell you how to do that.
You will also likely need to get an adjustable vacuum advance can and set it so it only adds around 6 degrees of advance once your distributor curve is set to the optimum level for your particular app.
