340+ for the STREET

Build what you want. A 340 with OD will need to be built with torque in mind, and 340s are not really built for big torque. Strokers will get you torque, but cost more to build. Mileage wise if it's built right, and the combo's matched, you can get close to 20mpg in a A body with 3-series gears and an OD trans and a 4" stroke. I know E bodies with another 800lbs+ with no OD and auto that get over 17. Octane need is strictly based on engine component choices and design. Has nothing to do with displacement. So get with a builder that knows what you want and how to get it, and get all you want.