Timing COULD have an impact on the 'run on' in that if it were too far off and causing extra heat in the combustion chamber. Especially if the timing is too late. And it is worthy to note that the 'bigger' the cam, the more ignition lead that they need/want. Stock specs will not work with hotter cams.
Triple 2 barrels are usually a bit much on a mild 383 and will only work if properly tuned for them. Personally, I like the six barrel set ups. But, they can be difficult to dial in the first few times a guy tunes them. Rarely are they a 'bolt on and go' kinda deal. For guys that do not want to learn to tune carburetors, 4 barrels are a much wiser choice. And 4 barrels will not hamper performance. In fact, since a 4 barrel is usually closer to what the engine wants and therefore makes a happier engine, performance often increases.
And it might be that the reason that you are only noticing the problems in higher gears is because the engine is under the most load there and that is where more issues are exhibited although they might actually be there all the time.
What rear gears do you have? Since you don't have the engine's build specs, what is your cylinder cranking pressure? What is your idle RPM? What is your timing like; initial, curve and total?
LOTS of things can play into your 'running on'.
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