Can you let off the throttle slightly and clear the smoke without affecting the power?

What this sounds like to me id the IP is set higher than stock, and at lower engine RPM it is simply overfueling the engine.

With a mechanical IP, you can only get a limited amount of fuel curve as the RPM changes.
Part of why the electronic fuel injection Power Stroke was mandated by the EPA to meet emissions.
The computer controlled injectors, where the computer is taking speed, RPM, engine temp, air temp, boost pressure and throttle position into consideration for each injection event is capable of much more fuel adjustment.

The IDI only considers engine coolant temp above or below 112 degrees, RPM and throttle position to determine how much fuel to inject.

My fix for that problem, run 4th gear at lower speeds.
On my truck I find myself wanting to shift into fifth at 70 MPH.

A turbo would also take care of that problem.

Any chance the IP is set up a little on the fuel?
Or G code injectors were installed?

Also replacing the exhaust with 3" pipe from just in front of where the two down pipes come together will help with both air flow and let more heat out of the engine.