You can buy the amp guage for 99 bucks and you may want to check the connection at the coil and the tach wire under the dash.
If you wanna just change them out, send it to me. I can install them for free labor plus send me the parts. Check the fuse for your back up lights, that is also the power to the tach on a 70.
Uh, you don't have mice nibbling on wires do you?