The inline 6 cast iron manifold on my 5.9 L Cummins is a mass production unit. There is good benifit to evenly spacing the pulse feeding the turbo. You need equal length tubing for that. Divided turbine housing benifits from feeding half the exhaust to each side. The header I'm designing will send cylinder 1 2 and 3 into half and 4 5 and 6 into the other half making the header a true pulse tuned unit.
The max fe is generally at the same point as max torque. Any increase in ve will drive that point lower on the curve. I don't know if that benifits fe, but it will make more usable hp and torque. I realise this isn't your goal, but if the engine can breathe easier, it must have some fe gain. I can build myself a custom header for next to nothing, but it might cost you more than it's worth to have those built for your whole fleet. It is just food for thought. Another wrench in the fan for a custom header might be downstream emissions equipment like dpf, cat, egr or urea injection. My trucks came with none of that, but yours must be newer than my 98 and 99 models.
You can still do things to gain efficiency with your turbo. Put a blanket over the turbine section to avoid some energy loss (heat).
DodgeRam