The header to H pipe joint does not use a gasket, it is a metal to metal ball and socket joint. If you used any WD40 or other penetratiing oil on the exhaust parts, it will make smoke when you start the car. I hope you used lots of antiseize on the header to H pipe bolts. If you didn't you will find that they corrode and are hard to get off, just like the original bolts were.
A standard part for Mustang repair is a 1 lb brush top can of NAPA antiseize or Bostik Never Seeze with nickel. Use it on anything that has threads and it will torque down tight, not loosen up or seize. Best thing since the torque wrench, which is its companion tool.
A standard part for Mustang repair is a 1 lb brush top can of NAPA antiseize or Bostik Never Seeze with nickel. Use it on anything that has threads and it will torque down tight, not loosen up or seize. Best thing since the torque wrench, which is its companion tool.