This is from Jere Stahl's (of stahl headers) website. I don’t know if the site still exist. There was lots of helpful information about dynoing on his site. I copied it into my dyno book when researchinghow to build my dyno. Here is what he had to say about dyno exhaust. For what it's worth.
Wow, that is interesting and I'll pass this along to Tim because we've just been using 6 inch pipe, 12 foot long with 48 ×6 inch mufflers at the end with a slight bend from collectors to the ground which is about 2 feet