As long as you acknowledge everything, there really shouldn't be too high a degree of fuzzy. Intention counts for a lot, but of course, you don't always know the author's intention...

I have a fic that's a movie rewrite that I'm now hesitant to post because of all this plagiarism stuff going around. I don't feel I've plagiarised or anything, as I've fully acknowledged everything, but I'm more worried about the reception it will receive in the current climate.

Out of curiosity, why do you use the term 'fusion' rather than crossover?