do i really need 3" exhaust

Building a 408 for in my dart. Going with a good size roller cam, stroked, tti headers, tunnel ram and ported heads plus other mechanical upgrades. engine builder is recommending 3" exhaust, I was thinking 2 1/2". Going to be driving it approximately 1500 miles a year. Is 3" really needed of is that overkill? I'm worried about ground clearance plus I already have 2 - 21/2" flowmaster mufflers I thought I could make use of. What are your thoughts?
Thanks, Rod