What's the best way to fix aluminum intake threaded hole for temp gauge sending unit?

The factory senders are 1/8" pipe thread. You may be able to chase it with an 1/8" NPT tap. Expando, maybe red Gasola - the old varnish type will seal it. Lampwick is hard to do with 1/8 NPT. I would tap it to 1/4" pipe thread - 1/4 NPT and run a brass bushing with Expando or Red Gasola.