I have used the ford idler pulley, and although pricey, it is a quality piece and works well. Some people dont want belt issues, and would rather just run the idler pulley, this is understandable.
I recently came across another way to do this, and it is the best ive seen yet, looks great, and it eliminates the entire alternater mounting bracket (that huge hunk of aluminum), the tensioner pulley, and the smog pump, and it leaves great clearance so you dont have belt rubbing problems(because it eliminates the tensioner entirely). It really cleans up the engine compartment and works well. It is March's alt. pulley kit, part no. 1465, and is around $58.00.
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1993 GT/AOD
'93 Mustang GT