Stock 340 intakes

If it's not a daily driver and you wan to get an extra 1-2 MPG out of it and you drive it hard ... I say you lighten up on it and I bet you will get your 1 MPG as is. That way, if all your components are working for you (performance-wise), you will have that power on tap when you need it. If you can't keep your foot out of it, perhaps a dual plane and a 600 VS might give you the results you want (again, will be better if you practice keeping the pedal from the metal). Also, consider exhaust mods (I don't know what you are running), ignition mods, fuel type, etc. Also, simple things like correct tire pressure really make a difference. BTW: I agree with crackedback about the factory 340 manifold with a spacer (especially if you were not going to buy an aftermarket dual plane manifold).