I was thinking about what to write, when this comment popped up. I agree. The 4 cylinder idea, is really bad in my opinion, for all the reasons people have already stated. Even a 4.3...it's going to have significantly less power, and mileage is not going to suddenly be great. (I had a couple 4.3 Blazers, they got around 15 MPG most of the time, in lighter vehicles. My 454 gets around 10. I'm guessing you're right in the middle with your 350?)
I think the two best ideas are either buy a cheap economy car to putt around with and drive it into the ground, or approach the fuel economy thing from the hot-rodder's "every little bit" angle. Keep your tires inflated, slightly high even. Use a slightly thinner grade oil, improve aerodynamics, K&N filter and lower-restriction exhaust, and most importantly, keep your foot out of it.