You can do this with the tank in. A little lift may make it easier. If all you're replacing is the O-ring, you can probably get the old one off and the new one in without actually removing the sender. You're better off if the tank is nearly empty, because even if it doesn't spill out, the gas will float the sender up a little and make it hard to seat.
If the lock ring is rusty, grease it and the o-ring before putting it back in, and make sure that all three tabs engage before you start to turn it. It can take a couple of tries, and you may wish for a third hand.
If you don't have a brass punch, aluminum also works. Wood will tend to split, but might be possible in a pinch. Just make sure you don't use steel.
