I got the QA 66" on mine and use it all the time. Haven't found a load of clay or wet rip-rap buried in clay/sand, or anything that it hasn't been able to lift a full heaping bucket load of. I have a 72" BB on the back (550 lbs) when doing this work and have my rear wheels filled too. Every now and then with a heaping load of wet clay, it will feel light in the rear, but at controlled speed I have no issues at all. Just keep the load low to the ground while travelling.
I have dug out a lot of stumps, pushed up to 6" diameter trees over and pushed into the woods, and moved a lot of sub surface rocks. I amazed at what this thing will do. On some stumps, I'd get under them and sometimes wouldn't lift, but if you keep working at them eventually they come up.
I should have posted pictures of my last project, but I moved a lot of dirt and rock in no time, recontoured my 300+ ft ditchline by digging it all out, then filling in to make a nice wide U type ditch and sodded the section of ditchline 20' wide to get started with. Box blade and loader really do the job. Made a swale down one property line probably another 300 ft or more to direct run off into the new ditch and control run off also. Moved piles and piles of dirt. Before sodding, had to refill the other ditch line where I dug out all the sediment and rip-rap and moved more piles. Had a blast the entire time. The tooth bar is a must for this kind of work in my humble opinion and has been worth every penny. Boy do I love my L3800.
Ok, then why are you trying to put a 72" bucket on it? kubota lists the 66" as an option... most likely they don't think the tractor should have a larger one. I *think* that the QC is universal so if you get a larger bucket with the same QC - it should fit. I don't have one... so maybe someone with experience will chime in....