Shiloh, our 67 has a metal flange in the floor, and the bolts slip into it from the top: There should be no need to work from the bottom or to saw anything off -- unless they're just corroded together somehow. The bolts are generally made of brass.
Just a slight thing with ours. Last time I had the toilet off, I realized that part of the reason it was sooo hard to reinstall was the shortness of the bolts. So I located extra-long toilet bolts at the hardware store, and they certainly helped.
Oh, and be sure to use the rubber-style RV toilet gasket. It's flexible, allowing some down-the-road movement while maintaining the seal.