On a dead stock 340, most dyno tests have have seen are better than 30hp. Tno testhis of course is not only a low end report but just a matter of what parts were tested as such header design, tube size, collector length, exhaust pipe sizing and length are all factors along with what ever muffler you use. Most choose the sound of a muffler over what actually makes HP.
To this end, end build choices and the muffler choice, dependent, on how he mufflers actual ability, you could throw away 15 to 100 hp away.
While no one in their right mind outs a stock muffler on a tunnel ram build, I’ve seen idiot choices that are nearly as bad.
Research!