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Specific header choice depends on how many cubic inches your motor has, rpm range you plan on being at most of the time, OR sizing the headers for top hp gains at full throttle red line.

When I built my 418 stroker I had to chuck the SPF provided headers (1 3/4 ") for custom Doug Thorley headers with 1 7/8 inch diameter pipes mated to the 12 bolt flange at the muffler. You can expect almost any muffler sidepipe on a Cobra to cut hp by about 5% on most any street motor and 10% on a full race mountain motor. Keep in mind that stainless steel headers do not perform well over the years around the welds due to constant temp fluctuatiuons. AND they radiate copious amounts of heat if left uncoated. Stick with a mild steel header with ceramic coating of your choice. Talk to Doug Thorley headers in Calif if you are thinking about a custom header. I'm surprised Kirkham didn't have some suggestions. Maybe they did, but just not for SS?