Since the boost gauge is mechanical, a basic vacuum/pressure hand-pump w/gauge should help check/calibrate it. I've had some sit wonky and heard they act out if they get moisture/rust/gunk inside - old age doesn't help. Best to simply replace at that point...maybe donate it to target practice
I'd be more concerned how it reads in operation than where it sits when off. Might re-center the needle so it sits @ 12:00, only to find it's faking a reading when the engine is running. The SC's gauges aren't Rolex grade, so don't put too much trust in it
Fuel gauge is electric, of course, and as I recall, they don't zero when off.
I'd find a known good one first, to confirm that either the sender or the anti-slosh board on the cluster aren't root cause (be aware those daughter boards may look similar, but vary by year and need to match the cluster). The sender could be rusted and/or wiring at the connection (pump and sender are one unit) could be damaged, corroded, melted - may have hurt the gauge?. Does it read correct when full? How many gallons did it take when the gauge said 1/2? Might want to confirm no damage to the tank floor. Be sure no one tried to lift the car by it at some point in the past. Bench testing a fuel gauge might be covered in the factory manual, but you'd need a way to supply a calibrated load/resistance, including grounds to even get close, I think. If you can get your hands on a new pump/sender, you might be able to use the EVTM to help mock up a reliable jig on the bench... Just keep in mind it's very easy to wreck it when testing, so...
Is this the same car w/the questionable wiring in the engine bay? If that mess is any indication, there's no telling what's been done to the cluster - good luck.