You both should try Titian Sunflower if you can find the seeds. I think seed savers had them a few years ago and may still carry them. They are monsters in terms of flower head and height and are super fun to grow. I think you'll be surprised just how big of a sunflower you can grow with these seeds.
It's best not to transplant sunflowers if you want huge heads and tall plants. Transplanting stunts the plant, effecting both flower size and height. Covers are used to protect plants from frost in colder climate and the planting holes are heavily amended or seeds are sown directly in finished compost piles . Most contest winners for largest veggies are from northern states. The main reason is you get more hours of daylight in the summer than folks in the southern US and sunlight = photosynthesis= larger plants= larger veggies, flowers, fruit, ect... Believe it or not Alaska holds several world records for largest veggies and leafy greens mostly due to the extended days.
Besides Titian sunflower, Dill's Atlantic Giant pumpkin, and Carolina cross watermelon most prize winning seeds are rarely shared by the growers and good genetics are just as if not more important than daylight hours.