I ordered the short toilet from Vintage Trailer supply, and the threaded floor flange. This allowed me to "dial in" the height for my toilet. You thread in what you need, then put spacer boards screwed to the floor to keep the height and weight off the toilet.
However, in mine, a rear bath 64 Overlander, the new toilet bowl was a little bigger (thankfully) however, that meant to keep it in the bench, I had to cut a little of the back of the bench, and move the toilet hole back about an inch. That's a real hassle. I will also have to modify the flush pedal.
It just depends on how original you want to keep it. One option on the flush pedal was to reverse it. I thought about mounting a pulley to the floor, then a ring in the original pedal. Run a rope/cord from the pedal, through the pulley in the floor, up through a hole next to the toilet seat, then a ring. Pull on the ring to fill, pull further to flush. Very minimal impact to the surrounding. I have my original pedal, so I'm trying to figure out how to mount that to the new toilet so everything still looks as original as possible.