The alternator belt is a pain to put it lightly. The only way I got mine to behave was to replace the belt and alternator (my pulley was worn), and tighten with a pry bar against the top of the frame (tire irons work great for this!) You may have to retension in a day or two as the belt settles.
That said, do note that A/C has a different alternator belt than no A/C, as does the 70A+ alternators compared to the 60A and lower ones.
1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).