The new tank should be fine without washing it out. It's usually got plastic caps on all the openings so nothing gets in it. I usually jack the car up under the leaf spring bracket behind the rear wheels and put jack stands under the axle. Make sure you chock the front wheels. You shouldn't have to jack the car up to remove and install the gas tank. At least I didn't on mine. When I put my new tank in I was getting a slow leak around the fitting so I used teflon tape and it's been fine since.