Not sure about the plates, but we plant sunflowers here about like you would dryland corn, or maybe a little heavier...... 20,000 to 24,000 I'm in South Central Kansas
We have a Kinze planter with finger pickups...I think like yours. We have special fingers for sunflowers that work in the finger pickup units. Your corn finger pickups might work well enough for what you want if you don't want to buy the sunflower fingers. (close enough for government work) We seed sunflowers at about 20,000.
I'm from Canada, we plant our confectionary sunflowers at 16000-18000 and oils at about 20000. If you have an air seeder you can just use that,,,,we have given up the old row crop systems here. We are getting the same or better results solid seeding them. Hope this is some help.