The deck will work on an 800 series as it mounts the same way:howdy:. The front drive kits are only partially interchangeable because the driveshafts and axle shafts are different.
Regarding belly mower decks, 800s use a longer PTO shaft, while the 400s use a shorter shaft as the 400's swift-a-matic transmission ( and pto output shaft) extends farther forward on the shared frames. As deercharles stated, that's the only difference.
Other (front mount; ie. snowblower, etc.) attachments will also work on 800 tractors. That said, 400 series tractors used a pto/through-the-axle 13 spline pto shaft. The 800 series tractors use a 15/15 spline through-the-axle stub shaft.
So, if you picked up a 400 series tractor with a blower, you would need to also convert the 13/15 spline double u-joint to a 15/15 spline u-joint on the blower side. You would also need to find an 800/8000 series pto-to -front axle driveshaft.