Hey Roger! Years ago, when I first started making DB, I tried a cherry/lemon version I called "Le Mon Cheri" (you like that, don't ya!). I can't remember how much fruit I used (my notes are at home), but I think it was either 6 or 9 pounds---as you suggested for yours. They were dark sweet cherries, and the end product did not have much "cherry" flavor, even after back sweetening. In the end, I flavored it with cherry Kool Aid, and it came out great. One of my best friends claimed it was his favorate. I haven't made it again.
That's just an FYI. You may need more cherries than you think to bring out some good flavor. Since I've only tried it once, I can't advise you much further. Good luck, my man!