I got chrisevo's Mustang in my workshop. The fuel smell turned out to be a rusthole on top of the tank. Someone had a go at fixing it by filling it with lead but it let go around it. We have decided it is not worth filling it again because it will corrode further from the inside anyway.
New tank worth $285 plus $25 freight on it's way from QLD.
See....i told you it is a simple fix.
PS: Any other Mustang owner with similar problem may not have rusty tank, it could be just the rubber connecting hose between tank and filler come loose or seal around filler cap broken or missing.
Cheers.