5.3 to 5.7 or 6.0??

I currently have a 2000 Silverado 4x4 with 4inch lift and a 5.3 in it. I just purchased a Magnusson supercharger and meth injection stage 2 kit. I have the oppritunity to purchase a 2004 GTO 5.7 that has a bigger cam and ls6 high flow/pressure oil pump and double chain that just got back from the machine shop to be checked out. It made 358RWHP with a tune and headers. My other option is a 2003 6.0 LQ4 out of wrecked 2500 that I would most likely have to go through. Other things are a BBK 80mm TB, 85mm Granatelli MAF, airraid air box and TB spacer, diablo sport predator tuner. What engine would work best for some pulling of car trailer and normal driving conditions? Also with headers; i'm looking at some long tube headers and they say that the 4.8 and5.3 are the same, what is the difference between those and the 6.0's? Also can I take out the egr and air injection since i'm getting a custom tune for it or is it not possible? The headers come withthe optio of with egr and air inj. and with egr and not with air inj. and without both the egr and air injection. Any help would be very much appreciated.

I'd buy a good waranteed 6.0L if it were me. I'd pass on the others.