... obviously, you can't have both. If you want good gas milage, go with a straight six or a smallblock V-8... 289cid is a good choice. If you want to keep it original, try to go with the factory engine. (boy, wouldn't a 428CJ be sweet!) The 360/390s are still plentiful and have lotsa muscle if you cam 'em up with a matching street-strip intake, headers, good ignition system and just a touch of nitrous but, hey; it's your money...