I had a similar problem, these trucks mostly came with the "Short" water pumps, hence the long flat curved upper alternator bracket. I decided to run a "long" style pump and the brackets are obviously different, but the lower crank pulley wouldn't line up and belts would walk off the grooves. I finally went to a salvage yard and got the matching set of pulleys and brackets, seems the pulleys I had originally had only worked with the short pump, or got mixed up with other pulleys sometime ago....I got the entire set of pulleys and brackets off of a late 70's Camaro, everything works as it should now.
Hope this helps!
Ben
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"Persistence is the twin sister of Excellence.one is a matter of quality,the other, a matter of time"
“Serious Black” my 1971 Short Bed Fleetside Sold to
an outstanding man in Lees Summit,Mo. “Gone, but by no means,Forgotten”