Here are a few things that I ran into putting my new black tank in my 68 Trade Wind ( a nice new fiberglass one from Inland). Make sure of the position of the black tank before you cut the hole. I started with measuring the old one and thought better of it. Glad I did. Since I replaced the floor I had to cut a hole for the vent and toilet. I put the bathtub and rear wall plastic piece back in the trailer, that way I could get the toilet in the perfect location. Pushing the flush lever opened the bottom of the toilet and I made a mark in the dead center on the new floor. Cut the hole in the floor and then raised the new black tank into position. After a little tweeking to get it centered just right, I marked the location on the top of the new black tank through the hole in the floor. Sanded and glassed in the flanges for vent and toilet and all went back together. One problem I had was the coupler between the flange on the tank and the closet flange was too long and I could not get the black tank up against the floor. I could not find anything at the local plumbing supplies that would work. I finally bought a PVC coupler with male threads the correct size for the flanges. Cut the threaded part off with my dremel. Because the pipe fitting has tapered threads, I then had to "square" the sides on my sanding table. Doing this will flatten the threads on the bottom part so I then had to reshape the lower threads with my dremel tool. Took about 45 minutes, but I now have a short threaded coupler to connect the tank to the closet flange and it is short enough to allow the black tank to be against the floor. Finally, I shimmed the bottom and the sides of the black tank with rigid foam insulation board so the black tank now fits just right into the galvanized box.
Hope this helps.