I'm debating on using spinach or Tuscan Kale as a stuffing for roasted chicken. Do you think Tuscan Kale would work well?

I agree with Susan. A gratin of spinach and perhaps onions? baked alongside the chicken will have more character. If you do stuff the chicken and use spinach, it will be awfully dense, which will make the chicken take much too long to roast. Alternatively, you could spatchcock the chicken and stuff it under the skin with the spinach (blanched first) similarly to the way I did with pistou here: https://food52.com/recipes/26593-crispy-roast-chicken-stuffed-under-the-skin-with-basil-garlic-puree

The problem that I find with stuffing chicken or turkey is that to ensure the stuffing is thoroughly cooked (165°), you cannot avoid overcooking the poultry.

I think some sort of grain and kale side dish baked alongside the chicken will make you happier. You can baste it with some of the drippings from the chicken to add flavor and the top will get nice and crispy.

Are you actually stuffing it into the chicken or do you mean alongside? I feel like if it is going into the chicken it will need some sort of starchy support - a grain or bread, to help it keep - what's the word - body? So it's not just a leafy green steamed on the inside of a chicken?