I would have to agree with geeteeohguy and recommend the factory ram air/ho headers. I had nothing but clearance issues with the headers I put in my 326 Tempest between the starter,frame,steering coupling and the list goes on. In and out numerous times to get the clearance required via malitizeing and using a mini sledge pounding,hammering,cursing,slamming,banging more cursing,tweaking,pounding and on and on until they finally fit.I loved the sound (check out youtube /65 Tempest Restoration) for sound on throttle up but was not worth the hassle. I'm now converting to a 4 speed and now have issues with the z-bar clearance so they're coming out. I have the original manifolds and hope to achieve the same throaty tone with a more aggressive muffler.Seeing that your dealing with a GTO I would highly recommend you check out the factory headers at Ram Air Restoration. Maybe a bit more money but you'll be able to get that great header sound and save yourself a host of clearance and heat issues and keep your ride closer to factory original.
Best of luck and keep us posted