RPM's

Those short tires are a mileage killer since like Rob said the final drive ratio is so low. Either put 3.23's in it or much taller tires on it. The 3.23 gears will help the mileage more than the tires cause taller tires weigh more so the drag from them is greater. Taller tires have the advantage of getting better traction but if that's not a problem I'd go with the 3.23's. Naturally any taller gears (or tires) will cost you some take off umph so keep that in mind. The converter, if it's a decent one doesn't really hurt the mileage as bad as some think as long as it's not slipping bad. If you go with a stock converter you may have trouble getting it to idle decent in gear at stop signs with that cam plus it'll lose a ton of take off.

Like Cannucky, I get far better mileage from my stroker but I also run 3.55's with 28" tall tires. When I keep my foot out of the 750 double pumper it can knock down 16 mpg. Even gear heading it I rarely see less than 12. Last fall I drove it 40 miles to the track, ran twice, drove home and checked the mileage and it got 14.5 mpg. Ran a 12.24 in the 1/4... As you can see fair mileage is achievable even with a hot car.

Even though your plugs might look ok on color the air/fuel ratio could still be off enough to affect MPG quite a bit. Install a wide band O2 sensor and you can watch what's happening. I put one on my Cuda and tuned the carb and it now gets 4-5 mpg more than it did and it's faster. Timing can make a huge difference in fuel mileage so if it isn't dialed in do so. If your distributor doesn't have vacuum advance get one that does. Shoot for 20-22 degrees of initial timing with 34 total and about 15 degrees of vacuum advance at part throttle. Tune the timing first then the carb cause timing affects air/fuel ratio

The suggestion of installing a OD trans sounds great until you look at the cost. If you drive a lot (at least 8000 miles a year) you'll recover the cost of installation in a few yrs. but if it's a weekend cruiser/hot rod it could take 15+ yrs. to recover the cost. I'm not knocking it, just looking at it realistically.