I have done this repair on our 1971 Tradewind . You have to remove the toilet first. Nots and bolts hold it to the floor flange. The tricky part is that there is a flange under the toilet that you need a special tool for. The flange ring is threaded and you have to rotate it. You will want to tap it with a hammer and screwdriver , but don't do it -you will wreck it! I measured across the flange and had a piece of steel marked for two "blades" welded on. This way you push straight down on the flange and rotate your homemade tool. Worked like a charm.
After the flange is off you can take the rest of the trim off the above floor tank and remove the tank.
BTW. Although the tank didn't leak, I thought this thing is old and eventually will leak and it is a big job to take it out again. I took the plastic tank to a local metal fabrication place and had it reproduced in stainless steel. It worked great and that was nearly 10 years ago. Good luck.
05 Safari, Artstream SE
1978 Olds 98
56 Ford Fairlane