68 Barracuda: temp gauge to max with key on

The only fault that would send the one gauge to extreme is a short to ground on the sender side of that gauge. Go under the hood and pull the wire off the temp sender, turn the switch on again. If that gauge goes to extreme again, the fault is probably at the contact pin on the panels circuit board. The pins can get pushed through and short to ground on the housing behind the board. All 3 gauges are operating on the same voltage limiter so if the fault isn't repaired the OIL and GAS' gauges will stop operting eventually.
These panels can be difficult to R&R but with practice and a few minor changes in the proceedure it can be less difficult.