In a Cummins application I believe I would lean toward using the upper plenum design pictured feeding a fabricated shelf on the head, rather than a individual runner for most applications.
With typical cummins rpm the runner length would become a problem because of space. Alot of work to do if done right. You won't see much difference of overall power but increase under the curve. There is a thread on a Nissan forum i believe for a machined Lehman intake that has a ton of good info engine was a 6 cylinder aswell. Long thread but a good read.