Presuming the fuel pump push rod was replaced during the rebuild? That the issues at and above 55mph did not exist before the rebuild? If you put the car in "2" and drive it, does the same thing happen at a lower road speed than in "D"?
What about the distributor point dwell? Is the point gap in spec when the point dwell is in spec? If not, then the breaker cam on the distributor shaft (what the points rubbing block rubs against) is worn and the dwell being flaky might result in a weaker spark. Only cure would be a different distributor or an electronic conversion kit.
How original is the muffler? Kind of sounds like a restriction might be causing a flow issue?
With the new carb, I'd look more toward an ignition issue. As some ignition and fuel issues can act pretty much the same.
Now, as to how it is acting, is the "sputtering" at a steady cruise speed or is it more on slight accelerations? Or part-throttle accelerations where throttle input approaches WOT but not automatic downshift speeds?
Am I giving the carburetor "a pass"? No, as I suspect the metering calibrations are decent for that engine size. If it was very lean, it would not respond as well and end up popping-back as engine load is increased. At those rpms, the main system is running things, with NO input from the idle mixture screws.
The muffler? Might a baffle have come loose and moved such that higher flow rates are compromised? It used to be considered that if such was the case, a rattling in the muffler could be heard if it was hit upon with the car not running, on an overhead lift.
Please keep us posted on your progress.
Just some thoughts,
CBODY67
