The 6915 fit without any issue, no dinging or denting of tubes required. The 6921 look nearly identical in the way they are fabricated, so I'd venture to say they won't have any issues either. The 6915 are honestly some of the easiest headers to install. Both sides go in from the bottom with plenty of clearance. The only easier header I've installed is the repop Tri-Ys designed for the early cars - you can almost throw those into the engine compartment and they'll install themselves.