the muffler will be fine either way as long as the exhaust dumps outside the body not under the a floor. My first build I just dumped the exhaust after the muffler and at 60-70mph in OD the thing droned like a Mo-Fo under the bed so bad I had to take it out of OD after 10 minutes of driving.My current build the exhaust will dump out the sides of the box just in front of the rear wheels.The mufflers are under the cab floor and are Dynomax Ultra flows and all I hear is the exhaust at the tips.
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