If you are shopping for a new bush hog, they come in 3 toughness ratings perhaps kind of.
Economy is made of thinner steel, lighter gearbox, just generally cheaper. A homeowner tool, they bog down in heavy grass, and giving them more power will cause things to break sooner rather than later. Might be a good value and good machine for many of us, but just know they are generally a lighter machine.
Mid range has thicker steel on the deck, a stronger gearbox, will hold up longer to rougher conditions, weigh more and cost more of course. Probably a long lasting, proper machine if you get into saplings and heavier grass now and then, want it to last the rest of your life.
Then the tough, extreme mowers, they are made stout and will last a long time. They can be quite heavy and might have to make sure your tractor can lift them! Premium gearbox and good bracing and all throughout. These will cut grass over the top of your tractor all day long, and laugh at good sized saplings and bounce over concrete blocks with little damage.
An 800 series Ford probably doesn't need the heaviest duty. A 6 foot is pretty big in heavy thick grass, a 5 foot is a little small if you are trying to do some pasture clean up mowing in a hurry, you probably want to look at a mid-duty 6 foot mower and adjust quality and size from there depending on what you are doing?
Your ~40hp tractor will do fine with any mower with a gearbox rated 50hp or bigger. If you find an extra heavy duty 6 foot mower with a 75hp gearbox, your tractor will run the mower just as well as it runs a lightweight cheaper 50hp gearbox mower. The hp rating of the gearbox is more about how tough built the mower is, not a rating of how much hp is needed....