If you're going to retap be sure to get a pipe thread tap. It'll say nptf on the end (exp. 1/4-20 nptf tap) and to get a matching pipe thread bolt. For a proper bolt you may need one machined (maybe $50) which will be cheaper and easier than replacing oil pan.
what makes a pipe thread special is that the threads taper and are much sharper and more accurate, which creates a seal. I'm 99% sure this is your best solution. I'm a cnc machinist and do a lot of pipe thread work. The reason a regular tap won't work us because it won't make the seal proper because they're less accurate ( + or - .005") and the straight threads don't get the sealing power of how the tapered Hole/threads and tapered bolt will.
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