PaulS1950

If you are running rich low in the rpm's and it gets leaner at the higher rpm's I would look into a different tune on the carb. It may be borderline big enough but the jetting and power valve are off. It should run just a bit rich at idle (not enough to smoke for sure) and then when you open the throttle it should richen up as soon as the vacuum drops and your power valve opens up. It should stay open and rich until you clost the throttle. At any cruise your engine should be running on the lean to neutral side of the mixture but never rich above an idle while maintaining your speed. You definitely need to look at the tune on your engine and your carb.
If the engine is running rough above a certain rpm then I would look at all the above and valve springs. If the can you are using is a fast lift cam it can cause the lifters to pump up before they would normally. Did you get the springs with the cam?

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66 Mustang Coupe. 365hp, 4spd Toploader, sub-frame connectors
Shelby drop, suspension tuned for slalom and hill climb
body mods and weight reduced to 2000lbs; name: Muskrat