I assume this was new. I'd have the dealer come out and pick it up before I started using it. If you use it and then decide you want it to be level, then you may have a hard time proving that the bucket was off from the beginning. For whatever you paid for the tractor, you should have one that was put together properly.
I would contact the factory rep on the phone and follow it up with a letter to the rep and a copy to the dealer.
Even 5/8's of an inch is 5/8's of an inch too much. If you wanted one not properly lined up and bent all to heck, you might as well have bought a badly worn used one and saved yourself a lot of money.
The dealer expects payment in full for a factory fresh tractor and loader, you expect a factory correct tractor and loader.
If they can't fix it, insist on a new tractor/loader combination. Your neighbor should have done the same thing when he took delivery on his Kubota.
Joe