RE: Band vs Broadcast Fertilization?
Banding it in is best if you can - but don't get too worried about it if you cannot.
For a band to be best used - it should be set 1-2" to the side of the seed band and 1-2" down in the soil. A localized band can allow you to use 1/2 the fertilizer you would use if broadcast - and its available to the seeds planted only - for the most part - at germination.
If you cannot band in the Nitrogen - You might want to plant it with none - or little fertilizer - then broadcast Urea or another nitrogen based fertilizer once the sunflowers are near knee high. This way - the sunflowers are best able to use the N put down - and can outcompete any "volunteer" wild weed seedlings that will have emerged. Little Nutrients are needed for germination - anf almost all soils will contain enough residual N-P-K for good germination. If you broadcast at seeding - all seeds start off with the same benefit - and weeds like ragweed, pigweed, foxtail, and others WILL outcompete sunflowers for the N.
Manual cultivation - with a two row - 3-point hitch cultivator - will help alot too. If you an cultivate - Cultivate the last time when the plants are knee high about - then broadcast 50-100 lbs/acre of N - and you will have a GREAT sunflower plot on most soils.