1986 converted to 5.0 runs like sh##

that is what it sounds to me is that one of the vacuum lines isn't fully seated or was forgotten to be placed back on. Re-Trace all of them first, pull each one off and re seat it at both ends if possible, doing this lets you look at them to see if any of them need replaced due to age also.

And Welcome to the forums. Post a pic of the stang if possible, we love to look at other peoples rides