Your car's VIN and the build date, are very important in determining answers to your question (almost all for that matter). I have two Oct 65, San Jose Mustangs that "bracket" your car's build date. Both have a dark color primer (grey?) used on the under carriage. Look for areas under the car that were primered but covered, like the ring around the seat access plug holes, or behind the tie down brackets, or the frame rails at the rear engine (aka transmission) mount when it's removed.