The throw of the sending unit arm has to match the depth of the tank, however in most cases they're adjustable. You MIGHT have to replace the gauge as well.
Fuel gauges are pretty simple, which means there's not much to go wrong. My guess the one that doesn't work may very well be the victim of a loose or corroded connection. The one that won't read correctly is probably a matter of adjusting the float arm (you would have to adjust the new one anyway). Probably with a bit of tinkering you can fix 'em both.
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw