Scenario: rebuilt 58-59 235 engine with 2500 miles on rebuild,30wt non-detergent oil, lean jet on Rochester carb, 48 suburban driven at 5300-8000' elevation, good compression. Symptom: engine runs fine when cold, begins to run rough when fully warmed up at 55 mph. Pretty much always at the same distance from home. It sounds louder from the exhaust pipe and the engine vibrates a bit kind of like running on only 5 cylinders but it actually has pretty good power. I originally thought ignition but now I'm leaning towards valves. It sounds like excess pushrod to rocker arm clearance except this engine has hydraulic lifters and makes no such noise when cold. I've already done a valve adjustment using companion cylinder method. Defective lifter? Sticking exhaust valve? I checked the exhaust manifold thermal spring when we started with our winter-blend gasoline here and my carburetor started vapor locking on me- at 65 degrees outdoor temperature! The spring is sticking the diaphragm in the manifold in warm-up position. I detached the spring and drove the truck with the manifold diaphragm wide open and it still had no effect on the rough running although my carb percolation problem greatly diminished. My best guess is an exhaust valve sticking in the valve guide that fully releases when the engine cools off.
So, engine detectives, where do I go from here? What clues did I leave out? Thanks!

1946 Chevy pickup, 1948 Chevy suburban, 1950 Chevy pickup, 1952 GMC pickup, 1957 Chevy pickup, 1939 Chevy pickup