Seems like the taller than OE tires and OD are getting that 440 out of its sweet spot at cruise. Agree a faster ramp low duration cam could help. Also, is there any opportunity to speed up ignition advance (mechanical and vacuum) w/o ping? That would help. Higher numerical gearing would help also with (maybe) some fuel consumption penalty. If the gearing gets the engine out of lug mode it could even help fuel consumption.

A really low duration fast ramp cam would help it run well in OD cruise but when you stomp it it would run out of breath pretty fast.