Three things that help a high lift/ high duration camed motor: Higher compression pistons, higher stall converter, larger fuel tank.

As far as the run-on, my next question would be do you have sugnificant power drainage from your battery if the car sits for a few days? Your ingnition system my be getting some voltage when shut off (rare, but does happen). The one time I saw this it was a wiring problem from the alternator.