If there's one bit of advice I can give you, it's what I was told back in the day (I'm only 20, can I say that?!?): Take it one step at a time (don't go big first off), and only modify one thing at a time. This will save you BIG headaches. No wondering if it's this or that going wrong. All right, daddy mode off.
I'd second the gears as a great first mod. Everything
Mad Horse listed is what you should aim for. Leave the heads & cam for later.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch