I did see a diagram which shows the bottom port having a plug with part number 9S-4185, but it was a bit confusing because the part shows "threaded" but the picture isn't. https://parts.cat.com/en/mustang/9S-4185
Similarly, for the top port I found a diagram with part number 9S-8005. Says threaded but picture doesn't match.
https://parts.cat.com/en/mustang/9S-8005
Here are the locations on the engine with the water heater:


Here are the corresponding locations with the plugs on the engine without the water heater:

