[Rory McNeil]

Does this Mustang have headers or the factory exhaust manifolds? Like Bob stated, 428 Cobra Jet engines did not use exhaust manifold gaskets, they were bare metal to metal. I realize that over the many years, it is common for rust a pitting to occur at the mating surfaces. On my own 428 CJs, I have had the exhaust side of the heads surfaced, as well as the exhaust manifold flanges. When bolting the manifolds to the heads, I applied a thin coat of Nevr Seeze to the mating surfaces, to help prevent later issues with pitting.
Now, if it does have headers, be aware that there are headers out there for Ford FE engines that do not have the proper exhaust port location, and will not seal up properly . The 58 up to about 1966 heads have an exhaust port location that is about 3/8" higher than MOST later heads, although the 68 thru 70 428 CJ heads retained the higher port location. Most 390 "GT" style heads had the lower location (except for the C6AE-R castings), so if the headers claim to be for a 390 AND 428CJ, theu will likely only fit 1 application properly. I have also found out that the Mr. Gasket header gaskets that claim to be for a 428 CJ do not fit properly, and because they have slotted upper bolt holes at the ends, the combination of the bottom of the slot (the 390 "GT" exhaust pattern has the upper end bolt holes located lower than the 428CJ heads), which combined with the higher exh port location, leaves less than 1/4" of gasket material to seal. With proper fitting header flanges on a CJ head, I have had good luck with either the FelPro header gaskets, as well as some more obscure companies like Ford Powertrain Applications (FPA), and Honest Georges. Ford Power Parts (FPP) also had specific 428 CJ header gaskets, but they have been out of business for some time. I have not tried them personally, but I have heard good things about Remflex header gaskets, however they are quite thick, which may cause clearance issues between the header flange and shock towers. Not much room there! The vasy majority of 428 CJ heads carry casting number C8OE-N, and have 4 bolt holes around each port, I always try to get as many header bolts in as possible to help with the gaskets sealing. Its an ugly job to change the header gaskets in the car of a FE Mustang or Fairlane, you sure don`t want to "enjoy" doing the job more than once.