Joseph Smith's translation of the Book of Mormon was more than Joseph simply reading words off of a seer stone, which is the basis of the "tight translation" theory. It was Joseph's responsibility to clothe the ideas of the ancient text in his own language. This would result in what is commonly referred to as a "loose translation" of the Book of Mormon text.
The best evidence for a loose translation can be found in Doctrine & Covenants Section 9, verses 7-8,
which was a revelation received after Oliver Cowdery tried to translate and failed.
"Behold, you have not understood; you have supposed that I would give it unto you, when you took no thought save it was to ask me," the revelation states. "But, behold, I say unto you, that you must study it out in your mind; then you must ask me if it be right, and if it is right I will cause that your bosom shall burn within you; therefore, you shall feel that it is right."
If Joseph were simply reading the text off of a stone, that would be the equivalent of having "took no thought save it was to ask me." Translation required effort and intellectual engagement on Joseph's part.
It's also worth noting that D&C 9 is the only contemporaneous document that addresses the translation process directly. Most records used in support of a "tight translation" come from long after Joseph's death.