Trick shift fluid is nothing more than Type F with better temperature stability. Simply put, that's a light oil with foam inhibitors, seal conditioners, and no fricition modifiers like Mercon. Sorry to bust anyone's bubble.
Don't worry about that little clunk sound. Perfectly normal for an automatic. What you're hearing is all the slack in the drivetrain being taken up when you put it in gear. Just be sure your U joints are in top shape, and that your ring gear in the differential is properly set.
Now if the trans is running HOT hot, then you need a cooler. You should have a cooler on there, anyways. Heat is about the only thing that will kill an automatic, and very quickly. The heat in your shifter is probably from all the air blowing past the radiator, engine, and exhaust pipes. Mine did that on my stock '79 Capri with a C4, too. Hope this helped to stomp your fears out.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch