Thanks for the response BobF. It was incredibly difficult to find advise on this problem, although it must be somewhat common. Our house was built in the 60's, so the flange is welded into place, replacing was not an option we wanted to venture into. The toilet was approx. 5/8" higher that the floor; too high, meaning when the toilet sat on it without a seal, it didn't rest on the floor. We finally got lucky and found someone that could help (after visiting every home improvement and plumbing place we could get to). We added a marble "platform", about 1/2" thick, thinset to the top of the new ceramic tile floor. Toilet is installed and working well without rocking or leaks.