It's not necessary to disconnect the IAB using the feeler gauge method since it's done with the car off. Disconnecting it is a good way, however, to test to see if the IAB is working. Doing the engine running EEC test is also a good way to check.
My car is definitely not stock and I have the dreaded E303 cam with 110 LSA but my car idles rock solid at 650-700 rpms (It's lopey because of the cam, but definitely not rough). I use the feeler gauge method to set it.
From what I've read, the IAB disconnection method is for the 4 cyl. I tried it, though, before I knew that and didn't work that great. Some people swear by it. You start the engine with it connected and then disconnect it when ready to set idle. Set the idle to 600 rpm using a digital tach on multimeter, not the dash tach. Reconnect the IAB and then reset the computer.
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351W 89 Mustang GT Convertible