I hear that! I have a spare 455 sitting around, but since I already have one 455 car I think I'll keep the 350 in there for gas mileage.
When I got the car, the first few tanks were under 12 MPG. I have played with the timing a lot (springs, vac advance, etc.), and on the last tank I got over 15 MPG.
I also have a 200R4 up my sleeve, and hopefully that will be gong in in the next few months. I expect the lockup convertor to help a lot, and the overdrive to help a little. I already have a 2.73 or 2.78 rearend (couldn't count exactly), and I think the OD will help a little, but not very much.
-Bob Cunningham