Installation went like every set of headers that I have installed in my automotive life. By that I mean that exhaust pipe bending and replacement was required. As I am 68 years old, it is unlikely that I will ever install another set of headers in my life. I have never installed a set that did NOT require major work by the muffler shop to complete the installation. However, they addressed the performance problem I was seeking to correct. The unexpected benefit was a quieter exhaust.
5.3 2013 Silverado
I play proball and my buddy on my team has the doug thorley longtubes on his 09 6.2 silverado and he talked about it helping more with Power than even his Tuner. He wasn't lying. As far as installation goes. Just take your time and do it right. Take a day and do it right and it's not bad at all. Once you get the stock headers off putting the new ones on is not bad. Use PB blaster to loosen old headers. Much easier after