I'm pretty sure your going to have to have someone weld on a female pipe thread (bung) to match your oil sender..
Oil Senders have NPT threads, and you should have quite a bit of engagement in the threads to get a proper seal. Secondly if all is good with the threads on the sender, and the threads on the female part, then I don't believe you'll need any sealant? (Talk to someone else on that though, becuase most the stuff I do isn't changing temps like an engine would)
If you just drill a hole in the oil pan and screw your sender into it, I don't think there'll be enough wall thickness to achieve what your trying to do is basically what I'm trying to get at.
600 bucks? Dammn!!
RD