You'll be more than fine, it will be early on in the season. Lots of young birds. Inside 40 yds. which if the dogs are worth feeding you should be. I'd pick 7.5/6 in a mod choke personally as 5's usually do not pattern great in a 20, your experience may vary. Shot a couple truckloads of UMW phez with a twenty over many years.
Enjoy!

Chief