We had a couple of gallons in the tank too. If you disconnect the filler tube from the filler neck rather than the tank you should be able to lower it without any spillage. You can then just tip it out once it's away from the car or leave it half full and try and do the work with fuel still in the tank. Personally I'd empty it once you've dropped it and refill once back in place as it'll make the whole thing easier to handle whilst trying to refit