Contact Dave or Chris at ModernMusclePerformance.com They are trying to expand their classic car market and may give you a better price. In Feb. I ordered TTi ceramic coated headers and 2 1/2" exhaust (with mufflers) through them and they saved me $150 total from what everybody else was quoting me. That included accommodating my special order of `67 GTX tail pipes (vice `66 Belvedere tail pipes) and `67 GTX Stainless exhaust tips from Accurate! They made all the phone calls to the two vendors ensuring all the pieces would work (fit) together prior to ordering and kept me updated through-out the process. If I had questions, they always got back to me.
I'm guessing if you reference a complete header, exhaust system and tips order for `66 Belvedere II with a Poly 318, made in Feb. by a guy in MD, they would remember it well and be glad to know we are passing around the names of good businesses.
It's just another quote...I highly recommend giving them a call.
Oh! Go with the H-pipe. The older small blocks tend to respond better than with the X-pipe...and your ride is easier to work on should you need to.