[Kenth]

I have a set on my 1966 GTO. IIRC i had to remove some material on the drivers side manifold closest to the engine crossmember, no big deal.
I also added the solenoid heat shield and rerouted starter cables like they are on 1967 RA/HO GTO.
I don´t know about power gains, some say 5-10 hp at 5000 rpm´s, but they sound 100% nicer than tube headers.
I´m using a 2,5" Pypes exhaust system with 14" mufflers w/o X.

Once you have them on, you´ll wont look back.