The sending unit in a Chevy Chevelle is a singular unit within the fuel tank. The main concern when ordering a replacement, is the number of ohms for which the unit is rated. Chevrolets use only 0 to 90 ohms. Use anything other than that, and the gauge will not work. Different manufacturers use different ohms. The older Chevelles require a good, clean ground to the frame for proper functioning.
Siphon enough fuel out of the tank so that it is not over a half tank before lowering. When the filler tubes are removed, fuel will be lost when the tank is tipped, which is dangerous. Use a hose and a gas can to drain the tank.
Raise and support the vehicle on jack stands. Loosen the filler hose clamp at the fuel tank using a common screwdriver. Pry the hose off the neck of the fuel tank using a common screwdriver.
Place the floor jack under the fuel tank and raise the jack enough to touch the bottom of the tank. Remove the two 15mm bolts located in the tank straps between the rear axle and the fuel tank. Use a 15mm socket and a ratchet to remove these bolts.
Pull the straps down below the axle housing and remove the opposite end from the slots in the body. Lower the floor jack and the tank. Remove the fuel lines on the top of the tank using the common screwdriver.
Disconnect the electrical connector on the tank. Pull the tank from under the vehicle. Remove the bolts in the fuel sending unit on top of the tank using a socket and ratchet.
Remove the fuel sending unit from the tank. Insert the new sending unit, screw in the bolts and tighten. Plug in the electrical connections to the fuel sending unit. Install the fuel lines and tighten the clamps with the common screwdriver. Install the tank in reverse order of removal.
