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Maybe if the bracket isn't bent, maybe it needs to be bent in the direction to straighten that belt out.
You may not have a matched pump to bracket, they are not all the same.When I replaced my pump in my 74 K5 the replacement pump and reservoir did not fit the bracket properly. After getting the correct pump all fit fine. The differences are subtle but exist.Just a thought.
I ran into this same issue... And I simply just used a lot of spacers aka washers to get it lined up properly.
I thought I had a picture of what mine looked like but it looks almost like the shaft is too long on that pump.