They dimpled their tubes at the port to make more room for the header bolts and they parallel ground their flanges to assure a tight seal. Their patented Warlock bypass muffler won the SEMA best new product award for its revolutionary open or closed design in 1996.
Put these on my 64 gto, was happy with the fit, instructions say you may have to shim the frame mounts to make it fit properly but I only had to ding a pipe on both sides just a little, which I expected to have to do anyway, I've had some nightmare header fitment issues in the past with different vehicles, with how much i would have to smack em and bend them to fit, these just slid in nice and like I said just a small ding with a pipe on both sides, passenger side to clear a slight hump on the crossmember and a slight ding to clear the A arm bolt on the drivers side. Very happy to do only that for a very inexpensive but good quality header and not mention it's for a pontiac and not for something as common as a chevy
The fit was difficult. The motor needed to be raised . 2 bolts did not line up with the heads and required enlarging . The header interferes with the steering column lock rod.