Definitely risky to drill around it. I would do it if it was a brand new bit, and I'd be the most patient man on the planet … but still no guarantee's
Can you fit any sort of plug into it? I have a tank that was drilled once and I didn't need to use a sump anymore, so I inserted plugs into the threaded bulkheads with teflon tape so that I could just use it as a regular tank.
If this is an option, I'd consider plugging those off and drilling new holes where you want them. Not ideal, but probably less risk of crackle/breakage going that way. Assuming the original bulkheads don't leak I suppose.
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