Yea, they're really tight to start with (make sure to loctite the new ones when you re-install the thing), and they tighten up with use.
The best way I've found to remove them is with an impact driver. An air driver can work if you're good with it, but it can get ugly if you're not careful.
Don't worry, you'll ruin the screws. It's pretty rare to remove them in good condition.
Also, if you've had junk go through the engine, it's a good idea to remove the oil pump housing from the case and clean the area behind it. There's space in there where metal shavings can get stuck, then they float back out once the engine is reassembled and running. Not fun!
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PJ