On the mufflers, I'd looked for large cylinder type diesel mufflers, I'll see if I can find a past order.
I use adapters, some times stacked ( 2.5" to 4" to 6" ). (Custom solutions on my race stuff, when even .00005HP has value, and that .00005HP costs more than then 2 mortgage payments )
Its important to put a turndown right after the muffler. Because there won't be enough heat / energy / flow after the first pipe expansion to "push" out a tail pipe.
as exhaust flows it is dense and hot. So you can fit more "exhaust gas" flow in a smaller pipe. But once that exhaust gas is allowed to expand, it removes heat, energy and flow, like hitting a wall of air. So it now has less energy to run through an exhaust pipework. Basically, once the exhaust gas is allowed to expand, it won't return back without external energy input. So there should be nothing but a turn down off the muffler. ( keeping the same large pipe after, and running a tailpipe out back will also produce a deeper tone. Issue is the max exhaust pipe size that can be properly routed over the rear axle is 5", or 4" if you have factory bags. The achieve a deep deep tone, use a 4" diesel muffler, 4" exit pipe till it clears the axle, 6" section of pipe or large diesel type tip. But I always just put turn downs)
This is why hard core performance people wrap or put thermal coating on headers, and other close exhaust parts. You can see HP changes by just doing that. And the wider the temp gaps, the more the improvement. Example, ~3HP on a gas engine compared to ~8HP by using a blanket on a turbo engine. Because a turbo system will have way more "HEAT" than just a N/A gas system.
