Might be the sending unit, too, or something in the wires.

You can get an Auto-Meter (or whatever brand you like) gauge and hook that up, or there are people who can fix/replace the one in the dash:

[www.jcauto.com]

for example. You can also find the gauge units for sale on EBay etc, sometimes.

Might want to check and see if the voltage regulator for the gauges is stuck, if it is, it'll fry all your gauges except the ammeter. Pretty sure they still used that setup in '70, I know they did in '69.

Aftermarket gauge is likely the quickest, cheapest, easiest.

Good luck!
-Kle.