[Big Dave]

From the standpoint of being easily within the capabilities of a hot-rodder used to engine swaps, and getting the desired MPG I would:

1) Lose the gas engine and automatic
2) Find a GM 6.2 liter diesel A 6.5 is dimensionally the same but a bit heavier
3) Get the T-56 transmission out of a Camaro. Junk yards are full of them.
4) With the T-56 either a 3.08 or 2.73 rear axle ratio will work.
5) Upgrade front springs/shocks to accommodate the heavier (~700 lb) engine.

Bingo! You're over 30 MPG. Its mostly a bolt-up although you may have to fabricate new motor mount brackets.

You'll probably want to keep the engine NA, but if you have room, Banks makes a good and monkey-proof turbo kit for the 6.2.