On da real doe,
He needs to go with a good boxing trainer who'll instil basic boxing techniques. At his size, or any size for that matter, he needs to be working behind the jab and making the most of his speed & power.
He needs to avoid bulking up too much, he's a natural heavyweight - we don't want another Bruno on our hands (that's not really a bad thing).
McCracken is a good shout, great trainer and he's involved in the GB set-up, so that means quality sparring. However, he's involved in training close to 20 fighters so I cannot see him expanding his professional stable. We'll have to see what happens with Froch, once Froch retires I believe McCracken will train one of his former amateurs, hopefully that's Joshua.
Otherwise, I don't see anyone else over here for Joshua, a move Stateside would do wonders for his boxing ability and career.