This is kind of like asking how to change the main bearings in a engine without removing the oil pan.
Position, or carefully jack up the car so a corner of the tank close to the filler neck is the lowest part.
Then siphon the gas out of the tank with a 1/2 or larger hose. If you move the hose slightly on the bottom of the tank when it is siphoning, you can disturb crud close to the hose, and suck it out.

Here is a safer way to start a siphon. Put your siphon hose into the tank, below the fuel level. Put the other end of this hose in a container to catch the gas that comes out. Get a second hose, and put it in the filler neck. The second hose should not go in deep enough to go below the fuel level. Wrap a rag around both hoses, and try to seal them in the filler neck. Blow air into the second hose, gently.

There will still be a lot of crud in the tank. Go to a boat shop, or marina and get a marine fuel filter. They are much larger than automotive fuel filters, and many have a sump on them that catch water and crud. You will want to have the fuel filter in place, unless the car is being judged for a car show.

I would suggest you do remove the tank, to clean it out. It will be a hassle now, but it will save you having to repeatedly clean and change fuel filters, possibily when out on the road somewhere.

 

drain the gas completly. if it's a stock cowl tank, that will be easy, fill the tank with vinegar and let it sit for a day or so. being a light acid that will remove the rust but not the metal. it will also disolve a lot of the crud.. drain it out and fill it with gas, drain it to rinse it and refill.. you could also use a gas tank sealer b4 you refill with gas

 

Alrighty----broke down and did it the right way!!!!!!!!!!!!!

I pulled & disconnected the tank, It is FREEKING disgusting and nasty as HELL, exstreamly!!!!!! rusted inside of the tank.

The tank/cowl is already all painted so is there a way to clean it all up without hurting the paint?????

 

vinegar shouldn't harm the paintb