The temp gauge could be the sending unit on the intake. It sounds like the gauge works with 12v, I think it only needs 3-5, so 12 will peg it. Seems like your cluster isn't grounded properly. The gas gauge works from the resistance of the fuel sending unit, That wire goes under the sill plate, through the back seat area and under the car to the fuel tank. Since it works by measuring resistance, any bad spots on the wire can cause a problem. so it could the wire or the sending unit in the tank. Make sure the ground is good, then start troubleshooting the rest.
Chevy runs deep, don't step in Chevy.
chevy runs deep, Don't step in chevy!