Radiator Issue?

67 dart inst panel.jpg If you're going to put pockets full of coins into engine rebuild you'll surely consider more accurate aftermarket gauges. Meanwhile... working OEM gauges would be a great help.
There's a metal box thingy plugged into the back of your instrument panel called instrument voltage limiter. It wont break yer bank to take a stab in the dark replacing what is most likely cause of multiple thermal gauges inop.
It wont cure water loss at rotted leaking freeze plugs, etc.. but could prevent frying the engine beyond rebuildable.
Other than factory service manuals that are free downloads there are also ample related threads here and google images including the back of your inst' panel.