Installed Custom X Pipe Exhaust and Now Engine Overheats

Sounds like further diagnosis necessary. Without independent verification of the engine temperature, it could be any of the sender/gauge issues you raised.

Without knowing what else you may have touched while installing it, or where the welding leads were attached, it's tough to say if something else might be up.

Without o2 measurements, it's tough to say if it's gone lean. This could even be confirmed if you had ir temperature measurements off the exhaust before/after. If you were leaning out from the X pipe alone, you'd likely feel an unquestionable difference in the performance.