I need to take off my sending unit so I can hook up a manual gage to verify if I really have an oil pressure problem or not. Sender has some rust and I don't want to strip it while I try to figure out what wrench size i need...9/16 is too big and 1/2 is too small.

Is it metric? I noticed what looks to be another sending unit next to the oil sender, is that for the low pressure alarm? Can I take it out and tap into it for the manual gage? What about one of the fittings on the remote filter housing, can I tap into that for a pressure reading.

9/16. Yes you could put it in the pressure switch location. I alway use the port on the remote filter housing for the mech gage.