What size pipe.... a lot of people will tell you that the true duals will cost you power.. a lot of people really hate the flowmaters ( I personally LOVE the sound of the 50 series ).... What I will dislilke will be how the pipe will be bent, none of the muffler shops will have a mandrel bender.... another thing, I would put a X pipe in there ( put the 02 sensor in the middle .. if you have an 03 ).... it also really depends on your other mods and how you intend to run your truck.... if you plan on 300+ rear HP I would go to 3 inch but if not 2.5 or 2.25 pipe will be better for you. A 3 inch pipe will cost you some lowend torque and will not be worth it if you are not going to make it up on the big end.
To be sure this will be a personal thing and many here will probably flame my ideas... but I will iinstall a true dual myself and my times will speak for themselves...
the first guy with a Hemi to run in the 13.xxs back in 03 took his exhaust system completely off to run those times but he only ran it that way at the track ( you know it had to throw a lot of codes ) He also used empty coke bottles to put the rear O2 in and never threw a code, few others had the same sucess rate thought.
Good luck on your system, whatever you choose and keep us informed... I personally really want to know...
TJ