Here's what I would do next.
First lift the EGR valve pintle while the engine is at Idle. Simply reach your finger underneath and pull the diaphram upward, and verify that the valve is not stuck in the opened position. You should notice a very rough running engine while lifting the valve, that will smooth out again when the valve is released again.
The next step if not already done, would be to thoroughly check for any vacuum leaks that would be causing a slight misfire at Idle. Listen for what sounds like a hissing air type of leak, and even a can of carb cleaner spray can be used to spray areas of possible air leaks.
You can next Pull one plug wire at a time, and listen to the engine RPM change. If you pull a plug wire, and the RPM does not change as on the other seven, then you have isolated a weak cylinder.
And the last step is do a good compression test, to verify that each cylinder is functioning.
If you have a valve that is only slightly burned, and there is some compression in one cylinder, but the rest have more, this may seem OK, until you get up around the 30 MPH range, where that low cylinder compression may not be allowing for combustion in that cylinder.
Seems like that year big block should have about 150 lbs of compression, and if 7 are at 150 to 160 and you have one at 110, you may have isolated the problem.
I hope this information is helpful.