Once you pull the panel from the dash you'll find mounting /contact studs on the backs of the gauges. You're looking for 20 ohms resistance across the fuel, oil, water, gauges and zero ohms on the amp gauge.
Keep in mind that just removing the panel can break stuff. The contact pins on the boards are very fragile. Best plan is make sure its qualified plug and play ready before installing it.Otherwise you might R&R the panel repeatedly. At 40 yrs old, tests OK today doesn't mean still OK tomorrow.
Nothing about it that cant be repaired as good or better than new. Worst I've restored had been too close to an ignition switch fire.
That invoice totalled 248.00