Oil pressure gauge maxed out

The oil pressure gauge on our car is a glorified idiot-light, it show L if you have <~6psi oil pressure, and somewhere midrange if you have >~6psi oil pressure. Your sending unit (near the oil filter) might be shorted to ground internally, you may have a wiring problem (make sure all of those grounds are reconnected), or you may have a gauge problem. If I had to guess, it's a wiring issue after an engine installation. As you're running with no alternator and low voltage, you'll want to get that remedied before worrying about the gauges. They're prone to acting up when voltages aren't what they're supposed to be.