Update
It's been awhile since I started this sound test. Finally have the modified 12" X-pipe and the modified 24" Magnaflow mufflers installed. All went well with the installation. All of the measurements came out correct, so no cutting or welding was needed after the modifications were made to the shorter X-pipe. I will say that getting the SW 18" X-pipe off was a real bitch.
Results of the sounds tests (db's):
Exterior @9'
----------------Before---S.Works---Magnaflow
Idle----------- 60-------71----------68
2500 RPM-----74-------84----------84
3500 RPM-----81-------86----------87
Interior (db meter at instrument cluster)
65 MPH-------73--------75----------74
70 MPH-------74--------78----------76
80 MPH-------76--------80----------79
Accel to 80---88--------86----------87
Although I did GoPro the tests, no video this time. Didn't think it was needed.
Comments: At idle the exterior sound level is quieter than the SW mufflers. At elevated RPM's, the exterior sound level is about the same. At speed the interior sound level by the db meter was a little quieter at 60, 70, and 80 MPH. Acceleration to 80 the sound level was about the same. But, to my ears, independent of the db meter, the sound level was noticeable quieter. The interior drone was almost gone. The sound level, although louder than the Lightning exit dual system on my '01 F-150 (standard cab - not SCAB nor SCREW), is very doable.
With this change in mufflers and associated length of X-pipe, I may not need to install the Helmholz resonators. I'll drive it like this for awhile and see how it goes. I'm not ready to give up on the Lightning style exit system yet.
It should be noted, that if I was doing it again, I would build (no one makes it) a true dual system using 2.5" pipe rather than the SW 3" pipe size, as the original head pipes are 2.5" until they merge into the original factory Y-pipe. With a stock 6.2, 3" pipes are not needed.