Personaly I don't think the powerstrokes sound that great with straight stacks due to the v-8 motor configuration. The dodge with a cummins on the other hand sounds good just like a big truck. Thats just my opinion though good luck with what you decide, and I want to see pictures.
The guy around here that put them on his dodge's used regular 5" big truck stacks, it might of given his a deeper sound.
I am short on money right now, so I have decided to just go with a replacement 3inch exaust pipe/muffler... Will help power/torque out a little. After I save up some, I'll look into the stacks... Cause that would just be super cool!
I saw another Dodge with straight pipes the other night. It wasn't as loud as the 7.3, but it still sounded pretty good. I also saw a '94 F-250 N/A Diesel with straight pipes last night. It sounds stock at idle, but when that feller took off, it sounded like a regular gas engine with straight pipes except for the compression/combustion "trademark of a diesel" sound.