Re: fuel burn for 650cc pushrod (gas). I'm running one PG/PF engine at 680cc (750 class) and an engine at 647cc. We found less than 1/3 gallon gasoline burn per run (3 mile). This with 38mm Mikuni and 230 main jets, for your comparison.
If you make your tank (tanks) very easy to get on and off, it'll be easier for fuel check and for record cert. Food for thought; twin tanks means more connections and lines to route/cover correctly (especially for fuel class). We're doing fine on the MPS-PG-650 bike with a single tank offset to the left (meeting the 5 liter minimum rule). Leaves room for the coil pack on the right, and still get the rider low.
Hope this helps.
Regards, JimL