Well, first post in this thread by FFDP: "Run a .050 to .060 gap." So, answered 3 days ago. Then mguzzo answered your question about the gap "opening up": "even after 100K, the plug wouldn't erode that much." That one answered as well.
If I was a betting man, I'd say the PO didn't gap the plugs at all, he just took them out of the box, assumed they were pre-gapped, and .070 is just what they were at when they left the factory. Don't be the PO, gap them correctly (spec is .060, most of us tighten it to .055 because we have mods, some to .050, some even smaller for nitrous setups). Ideally, you want to run the widest gap possible (up to spec of .060, not .070+) without spark blowout. If you're so inclined, you can play with the gaps and see what your car likes and how wide you can go.
Not sure who told you headers were a REQUIREMENT for a smaller pulley, but that's no reason to just assume a .070 gap is ok when spec calls for .060. You're wrong because the damn spec calls for .060, google it if you want. I thought the gap specs were pretty much common knowledge.