OK, as of now, I have a 5" pipe on my truck. It's a temporary deal, I basically got a 4" to 5" adaptor and bought a 10 foot secotion of 5" pipe and ran it straight all the way out the back. (had to take one of the shocks off to make this work, in case you were wondering). Before that, I had a 4" exhaust to a single 5" stack. Plan was to do 5" from the downpipe back and then up to dual 5" stacks. The only problem with this is that there's no WAY I can fit a muffler in there with the 5" exhaust. So I ordered a 4" Dynomax Ultra Flow muffler and now plan to do 4" until it splits in the bed, and then goto dual 5" stacks.
I have only two questions about this. I never noticed a drop in EGT when I went to the 5" straight pipe so I doubt that going back to 4" with a muffler is going to hurt EGT's any? I just wanted a second, third, fourth,,,opinion. Secondly. Has anyone used the dynomax ultra flow mufflers on their 5.9's? I like the straight pipe sound but it sounds a bit unkempt, so I figured smoothing out the exhaust tone wouldn't hurt. The only other option I really have is the dynomax Raceflow? Just looking for opinions here?

 

You ain't gonna hurt EGT's by going to 4" pipe. 4" pipe can support up to I think 550hp before it starts hurting the EGT's. As for the muffler, I don't know about that. Haven't had one for almost 3 years now.