I'll be watching this one for sure, been trying to find a 4n1 for my 580B ever since I bought it about 12 years ago - NOBODY sells a 4n1 unless you wanna buy whatever it's attached to also -
The Case models are only about 80% of the capacity of standard buckets, most likely because the 4n1 has 2 extra areas (back of the clamshell and front of the "blade") that are beefed up for grading, grabbing, etc, plus extra metal to mount/protect the cylinders that operate the clamshell - so if you were to convert a standard bucket there's a good chance it would end up a few hundred lbs heavier than it started -
With a big enough tractor, might be a moot point.
After looking in vain for the last dozen years, I bought a 2-lid 6' grapple for mine - haven't set it up yet, too many other irons in the fire - but when I do, the whole loader will get a (not skid steer) QA setup so I can swap between bucket, brush forks, fork lift, grapple, and any 3ph implement I might wanna try out front - possibly including a hydraulic motor driven 6' flail that's now only rear mount pto driven - I never thought it was a good idea to first DRIVE OVER stuff and flatten it, and THEN try to mow it....
Anyway, from what I've figured out on my stuff I'd probably recommend KEEPING your full-size bucket and fabbing a slightly smaller 4n1 - not only for weight, but it's almost ALWAYS easier to design something from scratch KNOWING what it's gonna be asked to do, than to band-aid something that's ALREADY functional into something that's SORTA functional.
YMMV, but that's been my experience on most things... Steve