Correct me if I'm wrong, but if the back is square and level, then the front HAS to be square and level. The tractor is completely aligned by the rear wheels, and just floats on the front axel. I couldn't exactly follow what you were looking at with the level, but:
If your front axel is not level, and the rear of the tractor is, then the front wheels are different sizes. If the front axel is level (meaning the tires are the same size) and the front of the tractor isn't, then the whole tractor is twisted.
If the tractor itself is straight (and it should be - can't imagine what it takes to bend a tractor), and just the bucket is crooked, then either the loader arms are tweaked (might show up doing a cross measurement to check for square, if you can find decent reference points), or the loader is mounted incorrectly and one side is mounted higher than the other - measure mounting holes/bolts to bottom of frame.
Jay