It's a 305 loader (non-detachable arms, 60" quick-detach bucket). The metal thickness eyeballs at about 3/16", but I haven't measured it. I don't know if JD has an approved thumb grapple for it, but I'll sure find out. If so, I'd dearly love to have one, or an after-market version of same, but I have no idea what would be required to add the necessary hydraulic control valve.
In the meantime, I want hooks but I don't want to weld directly on the bucket. I don't mind drilling a few holes here and there, however. So I'm thinking of welding up a simple L-shaped bracket out of wide flat bar stock, something on the order of my 'engineering' drawing attached. This would allow a bolt-on hook installation (painted JD green, of course!) and using 3/8" flat bar stock should give me mucho plenty support across the top to avoid bending up the top of the bucket. What do you think?