Highway RPMS ?

When I was still using my 65 as a daily driver I consistently got 22 MPG on the highway running about 70 MPH. That was a well worn stock engine (225K+ miles), 185-80/13 tires and the factory 3.23 gears (are you SURE you have 2.76's?). I never had a tach installed, but would have guessed by ear that it was turning about 3000 RPM at that speed. Fuel economy started to fall off pretty quick if I tried to go faster. I would think the sweet spot for a stock slant would be 2500-2800 RPM. I would suspect the factory manifolding becomes a factor much above that. Also, remember that aerodynamic drag increases at the square of speed so at some point it will take a lot more energy to go a little faster. Again I think 70 MPH is about where drag really becomes a factor.

Lower RPM isn't always the silver bullet to achieve fuel economy. If the gearing is too tall you may put enough load on the engine to cause the metering rods/power valve to open during cruise (low manifold vacuum caused by increased throttle opening).

Also consider the increased rolling resistance and rotational mass caused by larger wheels and tires. This may hurt more than it helps.