If you enlarge the hole in the throttle blade, that will only increase airflow, and therefore will increase idle speed. What's your TP set at? Should be 0.97V or so; long as it's below 1.00V you're okay.
Believe it or not, Ford actually has/had a service kit to get the correct "plug" into that hole you're talking about. I dunno if they still have it available, but it might be worth looking into. Otherwise you could experiment and plug that hole with something, just to see if the IAB valve can hold the idle where it's supposed to be. Just my .02.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch 