Use the EPC test port. You should also ground the gauge at at the same point by using a large ring terminal and a nut to fit the 1/8" pipe threads. You will find this nut at a hardware store in a lamp repair kit.
Well. I didn't remove the battery tray. (Mostly because it would have been more difficult than getting at the plug from the bottom anyways.) I put the sensor in (the thread was indeed the same) and now all that remains is to wire up the gauges properly.