80 & 85 mph speed limits

The reason for my interest in speed limits is: I'm rebuilding the front end of my '72 Satellite Sebring Plus 318. When I install the de-stroked 340 I am building (305 CID) I plan to use a '68 factory 340 camshaft and keep the 2.71 rear end. The smaller motor will probably be happier cruising at 3000 rpm (84 mph) than the current 318 which is happy at 2500 rpm (70 mph).
P.S. I suppose I could also experiment with running in second gear (Torqueflite) at 3500 rpm (about 70 mph). That would be the equivalent of a 3.91 rear end ratio in "drive" by way of the math: 1.45 x 2.71 = 3.93. Dont know what that would do to fuel mileage though.