i did too once. but mine is missing all the emission control equipment. what i did is just ran the gas down to a 1/8 of a tank and then added about 4 litres of methyol hydrate ran down to aircare and passed with flying colours (they didn't bother to check under the hood) then got to a gas station right away and filled up the tank. i have seen cars with horrible exhast problems do this and pass. i do not recommend this course of action. but someone in the lower mainland may be able to help you with your problem.
check out the site:
www.vancouverstangers.com they are a good local mustang site