I have a goal of building an engine for my 2001 camaro that will allow me to get 50 MPG. I know most people don't think of a V8 when talking about fuel economy but I think this goal is achievable. When the car was stock other than about 200 lbs worth of weight reduction I was getting 33 miles per gallon on the highway without any special techniques. If anyone is interested I'll post about how I plan on getting 50 MPG without aero mods.
At least this platform is lighter than the current Camaro, so it makes your goal easier. Regarding engine, I'd keep an eye for a Vortec 4800 or a 5300.