Climb under the car and take a gas bath, good for getting that pesky dirt behind your ears off. It comes out the back side of the tank from underneath.
Yeah, it's on the lower front of the tank. The sender isn't hard to remove and you will have to crawl under the car in order to disconnect the wiring and fuel line anyway, so you might as well leave the tank in. I recommend running the tank really low first and/or draining it if it has a drain plug.
Put a clean bucket underneath to catch the gas, but make sure you don't get pieces of the o-ring in it. You will need to put the gas back in to get you to the gas station. Also, make sure you clean the seating area so you don't have a leak after you get done. All lessons learned.
The new sender should come wit a new rubber ring, not really an O ring, it's round, but as square shoulders. It can be tricky getting it to line up in the groove. The sender will have two tangs on it that are supposed to pass through matching cut outs in the hole where it goes. You may have to bend one or both of the tangs slightly for a nice fit. Once you can set it in place without too much interference, put that rubber seal in and you will have to manuever it into place with a small screw driver if the tank stays in the car. There is a groove it sould sit in when it's in the rigt place. Or, you can take a toothpick and run a thin line of RTV around the groove the rubber seal sits in and set the seal in place and let set up, then it will stay in place when you put the sender in. I'd pull the tank. If it's original, it will probably have sealent around the tank lip making it pain to pry up. On restorations, I put that roof rail foam on the underside of the tank lip and drop it in. Seals up great.
Oh, once the sender is out, get a pen light and look inside the tank. Don't take an incandesent drop light near the tank in case the bulb breaks...make sure it's not full of crap or rusted. You can get a new tank kit with drain, sender, bolts, filler hose for $99 when these smaller vendors as specials- Kentucky Mustang 1-888-861-0176
ok, they went up a little, still makes for a clean install.
As the lone dissenting vote, I found it easier to remove the tank from the car and replace the sending unit and float on the bench. I also had a very difficult time getting the new O-ring from the new unit to seat correctly. The tank really comes out easily in a '65. The tank is part of the trunk floor and comes out from the top. This pretty much eliminates most of the laying on your back part with gas dripping on your head.
Definately run the fuel as low as you can before doing this. If you have a couple of 5 gallon gas cans (the red plastic kind), what worked for me before I got a new tank with the drain plug, was to drain it using the rubber hose that is connected between the sending unit and the hard line. Leave it attached on the sending unit side, loosen the clamp on the hard fuel line side, slide it off the hard line and point the rubber hose into the gas can, keep an eye on it and if you have more than 5 gallons you can tighten the clamp back up to stop the flow, grab another can and repeat. Somehow I always end up taking a fuel bath anyway but this kept it to a minimum.
Naw overly complicated IMO. Drive the car till its almost empty, put the car up on 4 jack stands with the nose just a little lower than the rear, disconnect the fuel line and point it into a 5 gallon gas can to drain the tank then go inside and watch TV to warm up for an hour. Come back out, raise the front of the car another few inches to tip whatever is left in the tank back away from the sender and wait a minute or to for whatever was in the line to the front to drain out into the gas can and then replace the sender.