15 should be real doable with something that's tuned up well. I'd start with the carb, myself. Holley's are very tunable if that's what you have, and Quadrajet's are very good carbs for economy. The Edelbrock 1406 AFB I mentioned earlier is already set up for mileage out of the box, and has done well so far on the truck it's on, if you're looking for something you can more or less bolt on.
Whatever carb you run, one thing I'd suggest is to run the idle mixture as lean as you can comfortably deal with when it's cold. (Tuning with a vacuum gauge I normally go a little lean past max vacuum at operating temp.) I personally think the idle mixture plays a bigger roll in fuel economy than most would think.
One thing I also forgot mention before is fuel pressure. A crappy generic fuel pump can make too much pressure and push fuel past the needle and seat at times, causing poor mileage and potentially making it run like crap. I'd recommend always running a good fuel pressure regulator to make sure that doesn't happen. You could try that now and see if your mileage improves any, 5PSI or so should be a good number.
Also, another thing I'd think about is a manual trans swap. I don't know how you feel about that but thought I'd throw it out there. A lot of people run 700r4's and 4L60's, but I'd never personally put one in a truck I was going to keep for any length of time. A good Saginaw 4 speed, clutch, 3.07 gear, and Hurst linkage like the '66 I mentioned has will be less than the auto-tragic and will almost certainly outlast it several times over in normal driving. Burn up the clutch 100,000 later? Rockauto has the whole kit for less than $100, and it only takes an hour or so to change. A T-5 5 speed would give you your overdrive if you want to keep your 3.73 rear, and they are common and not that expensive. Might be something to look into.
I also think the four speed adds some extra "smiles per gallon", for me it does, anyway.
