The bolts you used look like they are SS not steel. Looking at the link you posted acklands says steel with a black oxide finish?
SS strength is not as good a steel and would ditch those if I were you. The different SS gradings can really vary in tensile strength and their ability to hold your pulley.
Both grade 5 or 8 will solve your problem. A good washer under the bolt will help as well.
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