Pulley bolts keep coming off

It could be they are worn but I doubt it, more than likely the new pump has holes that are made slightly oversize. A way to stop them from coming loose is to use some Loctite type thread locking compound. Remove one bolt, clean out the hole with alcohol or brake parts cleaner to remove any oil or grease, then apply the locking compound and torque the bolt down to 115 inch pounds and then do the next one. Let it set up and they shouldn't come loose again.

Most parts store will have a few varieties. Blue 243 should work for you. Buy the smallest you can find, it only takes a couple drops per bolt. Don't use Red unless you want to never remove them again, it takes heating with a torch to break that free.
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There has to be an issue with the holes in the pump flange then. When you put the bolts in are they tight or really loose? What brand is the replacement pump? Could you take a picture of the parts, maybe something will show up in those.

Are they going in with a tight fit or are they sloppy? The way they are installed they shouldn't even try to loosen as the torque on them is in shear not offset. Unless they were the wrong thread. What size wrench did you use?