Exhaust drone elimination

Straight through mufflers can be effective. They do tend to be louder at heavy throttle than a turbo style muffler with a bit harsher, raspier, and louder tone vs. the deeper, throatier tone of a turbo style muffler even if the volume is the same at light throttle.

These are generalizations. Muffler design is math heavy and precise and well beyond my capabilities.

Per the Flowmaster rep moving the deflector plates inside .050" can hurt the performance of the muffler!

Although I have made mufflers from scratch and some of them worked well the success was more by luck than skill.

Three chamber Flowmasters utilize a Helmholtz resonator. that is the third chamber.

When you blow across the opening of a bottle the tone you hear is the frequency (or one of it's harmonics) that it can cancel out if it was used as an Helmholtz resonator.

Helmholtz, to repeat, is a frequency-based device. It works best at ONE RPM level. Above or below, it's far less effective.

If you're gonna go with a Helmholtz resonator, you need to pick the RPM you want it to work at--I expect most folks would go with whatever RPM they're turning in highest gear on their favorite highway. Or perhaps whatever RPM the drone is loudest. Others might pick a lower gear, figuring that's the gear they'll be in while towing some heavy load. Still others might want more effectiveness at whatever RPM their engine's torque peak is at. Many choices. Pick ONE, and work the math for that engine speed.

Just like music, and just like two-stroke "tuners", if the Helmholtz resonates at a certain frequency, multiples of that frequency will also resonate; but lower in amplitude. If it's tuned to 3000 rpm, it'll still work--some--at half and at double that frequency.

+1 more for the Helmholtz setup, there is a picture of one on my 'burb in my build thread. It is the only thing that will get rid of your drone.

I had the largest case aftermarket straight through muffler you can buy, and it still droned. Helmholtz fixed the issue, when I pulled the cats after one died, it was too loud, now I run 2 Dynomax Super Turbos with the Helmholtz, perfect mix of sound, tone, and NO drone.

This is the 3rd vehicle I've used them on.

Route the exit outside: either behind the rear bumper or outside the body panel before or after the rear tire.
If you still have drone add a resonator or glass pack BEFORE the muffler.
You're running single pipe, which gives you an advantage over any other trucks running duals with the same issue: you don't have to run a crossover or y-pipe to reduce the individual exhaust bank pulses.
One of the nicest sounding exhausts I ever heard was a dual 2.5 into 3 inch single exhaust exiting out behind the passenger rear tire of a gmt400.
$.02