Be aware that although your tractor is advertised as 40hp, you actually only have 33hp at the PTO, assuming you have HST. In your case, I'd suggest a "truly" medium duty cutter of 5'.
Most brands have light, medium, and heavy duty. The only exception to this that I'm aware of is Brown Manufacturing, their lightest model is a seriously medium duty cutter.
I'd look into:
Brown Manufactuing model 415
Brown Manufacturing Corp | 800.633.8909
Landpride model RCF 2760
Land Pride
Woods model BB600 (NOT BB60). However, Woods changed their model designation and I do not know the current model numbers.
Bush Hog model 295 (they have also changed their model designations, stay away from anything that says "squealer" on it.
You can't go wrong with Brown, and that is my #1 choice. All the other brands have cheap, light duty mowers that are easy to tear up, however, as long as it's truly a medium duty cutter you can't go wrong with any big brands.
If you can buy it at Tractor Supply or other farm stores, stay away from it, it's likely junk on anything more than grass.
Thankfully, Everything Attachments is out of business so you don't have to worry about decoding their terrible naming conventions.