Jim,
I think you're running your engine WAY too hard! That sucker was never expected to maintain those kinds of constant speeds! If you look at the Torque specifications chart, you'll see that the 217.8 (218) engine's peak torque is at about 1200 rpm for '49 thru '53 and 1600 in '54.
You could really put an overdrive to good use, if you want to maintain those kinds of speeds. I believe if you attempt to use the torque ratings as a guide for gearing for highway driving, you'd be closer to the optimum drivability. Your ability to take off from traffic lights and climb hills may be hampered a little, but highway driving should be a lot less stressful.
I'd like to see other's comments on this subject.
