Got an 2003 dodge diesel, puttingon a 5" system and have access to a 5" donaldson muffler from Peterbuilt. It is not a straightthrough design, but was designed for bigger engines. Will this be a high flow muffler like a MBRP (compared to flow)

 

That will give you plenty of flow. Not as much as a performance 5", but I take it you're looking for better than stock flow but still quiet? Should be a nice deep tone.

 

I have a 4"x 51" Donaldson on my truck and it probably helped lower the EGT by 30-50*

 

On my Dodge I have a shorter style Donaldson that initially didn't go straight through, before installing it I cut out the center divider. I don't know if you have the same version but if so it's do-able.