I gotta agree - keep your engine in there, or maybe, maybe look for a 350 (if that's what originally came in your car, since the 455 isn't original). You can look to change the tranny out to a 4 speed - 7004R maybe? Or look into a manual overdrive system that can save you some gas with your TH400. But in the end, these cars suck down gas, and will never get close to 30mpg that ecomony cars easily acheive.

Oh, and $3.10/gal is pretty cheap, compared to the east coast prices.

Maybe drive the bird less?