Performance mixed with fuel economy!?

Last December I picked up a 1969 Barracuda fastback, original 340 car; fender tag, build sheet gone, so no documentation that is a formula S as the seller claimed. The 340 was gone, has a 360 that doesn't run. I've been trying to get it to run but no luck. I had dreamed of building a 340 someday. BUT. With the way the world is going, especially gas prices, I'm considering picking up a 318 instead. The rear end has 3.55 gears and is a posi. (Both wheels spin the same direction off the floor). What do you guys think? Anybody in the same boat?