In addition to a hushpower muffler just after the cat,what about installing another small muffler or resonator inside the 6" stack? fairly simple fabricate.Have the 4" pipe that comes thru the bed continue up thru the stack for 6 inces or so,weld a resonator or muffler with a diameter smaller than 6",slide the stack over your work,secure the stack to the bedplate.Anyone done this?

 

Not exactly. I have an Aero Turbine 5050XL (built in resonator) after my down pipe and a 6" single miter stack cab high. I have installed one of the in-stack mufflers in the stack. Very little difference, maybe a little less noise at cruising speed, but not much. They are cheap so every little bit helps.

With the Cat still on and a hushpower you are probably as good as your gonna get. Just my thoughts.

Pic of in-stack muffler:

 

Im just gonna leave like 12 inches of 4 inch pipe sticking up into the stack,if its too loud then i can weld some dampning inserts in the 4" pipe.