A bit of correction, it's not so much as the difference between 216 and 325, but more the difference between the Chevy engine and the GMC. All current production Fentons are mad without the heat riser on the rear header so that the same casting can be used on either engine. Otherwise, there would have to be 2 different castings for the rear header to make up for the length difference.
On a side note, I recently put exhaust heat on my Offy/Fenton combo (fabbed it myself) and notice no difference in how it sounds. But then again, mine doesn't have the "rap-rap-rap" sound, it sounds more like a cammed up Smallblock.
