are you after more free flowing to free up some HP which will be noisier or more restrictive to quiet your system some which generally costs some power? to ans your Q a longer muff will subject the gasses to more muffling which will be quieter assuming the longer muff will fit your vehicle. Just me I'd keep the Dynomax's (they're a good muff) and add some glasspacks alongside the rear leaf springs for addit'l quieting. with that plan make sure your choice of GP does not neck down where the end nipples are welded to the case