I've tried sliding it down from the top and up from the bottom, as well as a persistent email collection bar anchored to the top. All inferior in terms of form completion as well as revenue generated within three months for the cohort of people exposed to the form.

Haven't tried a block in the middle of the text, but that's because most folks are landing on my product page, and that's had a lot of testing to ensure it's optimized for sales, which are naturally more important than email collection.