Senders can be gotten from Napa. The ammeter does indiate charge, but using 1 underpowered for the job will cause a fire, maby not today, maby not tomorrow, but when the need for the alternator does go full feilding arrives, it will catch fire. to test the gauge accuracy, you need a known good gauge to compare to. water temp gauge is mechanicle most likely, so you just need to get water to the gauge w/ hose and fittings. gas gauge sender is standard early Ford, jut tell Napa guys, '51 Ford, they'll get you the right 1.