I just researched this same issue. I just purchased a FEL for my 70 hp Kubota and went with the Tach-All QA by ATI. I also purchased a Markham Grapple. Put this all together only 3 weeks ago. Awesome!! The QA is really simple, literally takes only a few seconds to remove or install the grapple or bucket, and it seems really solid. No way would I go with anything but a skid steer type. As far as your bucket, have you thought about just ordering a new bucket from Markham? I priced lots of buckets, and theirs was less than 1/2 of other brands. ATI and WRLong both quoted over $1000 for a 72" bucket whereas Markham sold me a really stout bucket for less than $500. They also discounted both the grapple and bucket, and discounted shipping for getting both. By the time you buy an adapter or buy steel to modify yours, you may only need to spend a little more to get a new bucket. I also purchased a tooth bar for a good discount. You may be able to sell your bucket for enough to cover a new one. Just a thought. Don't forget to order a bolt kit from ATI to attact the QA to your loader arms. My loader did not come with these bolts. You may want to research whether you need these also. I think these were about $90. What grapple are you ordering? I'm a tool junkie and I must say that this grapple is about the most impressive tool I have ever used.
Get the skid steer QA. You can't go wrong with it.