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Your ear-tach is about right. This car probably has a 4.10 rear axle, so it would be turning about the same revs as your 142 did in straight 4th (B20 cars with OD usually had a 4.30 rear) With one exception, this is not a recipe for rapid engine wear, but it will burn up a fair amount of gas (I had a 68 144, many years ago--B18, M40, 4.30--and it never did better than the low 20s, whereas my 71 142E with OD would easily do 30 on the highway)
The one exception is the valve seats. They are cut directly into the cast iron head, and are prone to rapid wear if unleaded gas is used. When you adjusted the valves (What do you mean about shims? Are you referring perhaps to feeler gauges?) were any of them, particularly the exhausts, way too tight? This is a sign of valve seat recession, which, once begun, will continue rapidly. The engine will begin to lose power again, after a few hundred miles, and the valve(s) will be found to once again have little or no clearance. The cure is to pull the head, and have hardened seats installed at a machine shop. New valves are probably a good idea at this time. The 68 B18B has 10.5:1 compression, and requires premium gas. Try to find an extra thick head gasket if possible, to reduce the compression a bit.
Once everything is back together, this car should be perfectly capable of the proposed highway commute, assuming everything else (with particular attention to brakes) is in good shape.
It may be that valve seat recession has not yet begun. However, it will eventually happen. Using a lead substitute additive in the gas, and driving gently will help delay the inevitable. I hope this doesn't come across as too discouraging, but it is the reality of using a 40 year old car as a daily driver.
It's a major job, but many 122 owners have added overdrive to their cars, which helps a lot with gas mileage, and makes it a lot more pleasant on the highway.
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