Hey 70imp I had this very same problem on a 71 chevy truck I built with a bb every time I stuck my foot in the carb and she torqued up off came the alt belt. You need to go to a slightly bigger pulley diameter this will solve your problem, if in fact the alignment is correct.It may sound tachy but I used an electric motor style pulley and bored the inside out and keyed it to fit the alt. shaft.I'm a machnist so easy for me to say but you could get this easy task done at any machine shop done for probably around 20 to 25 dollars and get the pulley at any hardware,or electric or farm store. Try it it well work. wjr