If someone could come up with the various gauge sender resistances "from the old days" as I know they had test instruments at the dealerships, it would be easy. The resistance for fuel is around, but I don't have it at the moment.
But having sender values for temp and oil would be helpful. It just is not that difficult to rig up a temporary power supply to check them.
By the way there are lots of aftermarket gauges that aren't much more accurate.
"All mechanical, all the time" used to be my motto. Had a SW fuel gauge with SW sender in my old FJ-40 Landcruiser. Never did get it very accurate, and you could yank the sender right out the top. Finally settled for "MT" The rest was a guess.