[Christ]

The problem with installing mounts upside down is that the vibration insulator won't work properly.

If you're going to fill it in, then go right ahead. If you didn't plan on filling it, you might be able to press the mount donut out with a jack and a piece of exhaust pipe, flip it over, and press it back in, so it hangs correctly.

Then again, if you're not concerned with light vibration, you could just leave it upside down, and that would be fine, too. You'll only feel it around idle, anyway. I used to have solid mounts in my Civic, and I was never bothered by the little bit of vibration I got.