Cruising at 1800 rpm in 5th gear requires an engine to be tuned for a fairly low torque peak. Your engine with the Indy intake and the 2 inch headers is tuned to run really efficient at 4000 or higher. A smaller cam will help but if you want decent gas mileage at 1800 rpm you'll most likely need to swap in a Performer RPM intake and maybe try some smaller headers.