I have 4.10's, a 3200 stall, a custom made shift kit, and a tranny cooler in my car......
The only problem I could actually see you having is the tranny getting hot during the summer. A cooler would help you out alot, and they aren't very expensive.......
But, no, you are not wasting gas, unless you just could care less about the performance....but if you have 4.10's in your car, that is most likely your MPG problem
But no, a 3000 Stall should be just fine, especially if its a lock-up
-Will