Your Mustang has speed density fuel control system. Unfortunately speed density systems are not at all tolerant of engine modifications. So unless your engine is stock, it may not pass exhaust emission standards. Since HC emissions are controlled by the EEC-IV engine control module on a stock engine, the catalytic converters may have failed. My experience is Ford speed density systems typically ran rich (high hydrocarbons) anyway. Also probably why speed density Mustangs had quicker quarter miles times than mass air flow controlled Mustangs. So unless the catalytic converters were in pristine condition the engines would barely meet emission standards when new. Clean engine oil will help bring down HC readings during an emission inspection. If the engine has a misfire from a weak ignition system or leaking EGR valve HC emissions will be higher too. There isn't one simple answer to correct high HC emissions as it takes diagnostic troubleshooting to identify the cause.