Sorry to hear about your clowns. I have to disagree though about him being like a bad puppy who ate your shoe and needs to be scolded
Well actually I guess your kind of right but IMO its different. I know you already understand this but he came from the ocean at a decent size and his instincts are going to be much stronger than a baby porc (another reason big fish sould not be taken from the ocean). He grew up as an opportunistic predator who had to eat when he got the chance. If he ever found "tame" clowns in the ocean who did not try hard to swim away, there was no way he was going to leave them alone because he had already eaten a few prawns. They are eating machines!
I feel really bad for you and really, really hope this doesn't change how you feel about Griff. In your PM to me I got a good feeling about how much you care for this fish and obviously want whats best for him. Being that he was taken from the ocean at such a big size, he deserves a good home in a big tank like yours. If you take him back, he will most likely end up in a smaller tank. You really can't blame him for doing what he knows he has to do to stay alive. He will slowly loose the instincts to a degree and actually get somewhat lazy, only ever waiting for you to feed him. Eventually only you and the prawns you offer him will register as food to him.
I have had the exact same thing happen a few years ago. I had a nice pair of black and white clowns in a 180G and added a 6-7" Mappa Puffer. He hit the water, swam about a foot and came across the female clown. One bite and she was gone infront of my eyes. The male soon followed. Then my goby, damsels (there is hope there) and every other small fish was eaten over the next few weeks. After about 6 months or so though I tried some damsels again and he never killed anymore. They were my clean up crew. I think mine was another case of a decent sized fish being taken from the ocean and the problems it can lead to. I have had several puffers since then but bought all of them around 2-3" and not one has ever killed another fish.
Please give him a chance. He really isn't bad, just doing what he knows. He will make a great PET, unlike most fish. I know your an avid dog lover and this is as close as it gets with fish! There is a reason they are called Aquatic Puppies.
Can I recommend you add a trigger fish or two? Ideal tankmates for puffers and just as much personality as puffers. I would go with a pair of Blue throats in your tank or a Niger. Stay away from the overly aggressive ones like Clowns, Queens or Titans. Pink Tail is another great choice. If you want to get really expensive a single or pair of Crosshatches would be ok in your tank. A 260G is just enough for Crosshatches I think.
Also look into a bigger species of tang (not too big of course) and maybe a grouper to finish the tank off. Be careful with the grouper though because you don't want to end up with one that gets too big for the tank!
Good luck!