Preheat the oven to 375ºF and cook the pasta according to the package directions until it's al dente (it will continue to cook in the oven).
Add the slivered almonds, water, lemon juice, apple cider vinegar, and salt to a high-powered blender or food processor. Blend/pulse the mixture, scraping down the sides as needed. Add more water, one tablespoon at a time until you've reached your desired consistency. Add more salt as needed, to taste.
Transfer the almond ricotta to a bowl and add the garlic powder, dried basil, and dried parsley.
Cook the spinach in a little olive oil over medium heat until wilted and reduced in size. Add it to the bowl with the almond ricotta and seasonings, and mix together until well combined.
Add 1 1/2 cups of pasta sauce to the bottom of a 9x13 pan. Stuff about 1 tablespoon of the ricotta cheese mixture into each shell and nestle it into the tomato sauce until you've used up all your ricotta and shells.
Pour the remaining tomato sauce over the shells, cover the pan with foil, and bake for 30 minutes.
Carefully remove the foil, top with fresh basil, and serve immediately.