Originally Posted by BillGrissom

"... runs strong but has a roughness at idle"

A restricted intake wouldn't hurt idle quality since that is the lowest airflow. It would only restrict max airflow, so would reduce max horsepower. If gasoline engine are a guide, some strive for smaller carburetor bores at low power, being extreme in the "spreadbore" Quadrajet and Thermoquad carburetors. I put an aftermarket Offenhauser "Dual Port" intake on my Chrysler 383 engine. It has 2 runners to each cylinder, a smaller one off the small primary carburetor bores. The advantage of smaller bores and intake in a gasoline engine is more swirl and better atomization, which isn't very relevant in a diesel engine, as far as I know, since the combustion mechanism is much different than in a diesel (premixed vs diffusion flame).