Ford Repair: ford 427 aluminum block, mustang gt500, aluminum block

Question
I have a 1967 mustang gt500 with a 427 aluminum block and a super charger and i am having an issue with the idle.  the car idles at 1700rpm and as a result continuously overheats.  This problem came out of the blue about 6 months ago for no apparent reason.  the car normally idles at 800rpm.  the builder of the car, unique performance, is no longer in business and when i originally spoke with a technician at the company about the idling problem he told me that it was not a small issue and i should send it back in the spring. Needless to say, i can't send it back.  Any thoughts on what this could be?

Answer
Hello Michael, Tough one here. The high idle could be caused by a vacuum leak or a timing issue. It could have jumped time- high idle and overheats. Does it crank over slow after being shut off for a few minutes ? Have you tried to retard the timing any to see if idle changes? When it acted up had been on a acceleration? showing what a gt500 could do? (my odd humor!)  Let me know Thanks Tony