I would special order a door that size and have the framing all made with it. I would suggest a steel skinned door with blown in insulation in the middle. The door could be a 6 panel door with a wood grain look that you could stain or paint. Will look nice.
you will not have any more problems with rot if you go this route. But, I would treat the wood jambs with a good preservative, that purple stuff. Especially the bottom foot or so.
Most doors in home centers are 32 X 80. You cannot cut down steel doors because you expose the centers. You can only cut down solid wood doors. But with your comments on sitting in water, I would steer away from any natural products.