DetroitDiesel71

Hey all, I am looking for ideas for my 1972 Brockway with a 8v71N for the exhaust system. Currently it is single stacked with some brand of muffler 5" in and out. I am looking for ideas on what to do? are the grand rock mufflers that are so common free flowing with little back pressure? The old two strokes dont like back pressure. I want to dual stack it for sure though. Would it be better to just put resonators under truck with stacks? Let me know. I found grand rock mufflers for $65.00 a piece and are 5"in 5" out x 10" dia x 51" length. Going to be doing project this week but am holding off buying until I do a bit of research. Thanks all.