Can't give much help on the best route, but live in the Black Hills and can give some unsolicited advice.
Please don't take this the wrong way, but it's intention is just to help make sure you get home alive and well.
Don't ride impared in any way in the Black Hills, even if it's just a case of too little sleep. It's a madhouse with so many bikers here during Sturgis and tourism is way up already this year which suggests lots more folks will be here during Sturgis as well. Some areas of the Black Hills have really tight curves, so know your limits and stay within them. I tend to stay home during the rally, but almost every trip out I encounter bikes and riders who went off the curve or hit a deer, etc.
Speed limits are usually just reccomendations to those who are expierenced riders, but there is so much wildlife in the Hills that I reccomend 5 below all speed limits and always be prepared to stop. If you see deer, be prepared to stop as they often just stand and stare at you until the last possible moment, then charge acroos in front of you at the last second. I usually stop and let them cross. Same with turkey, they will fly at the last moment and hitting a 50 pound bird at chest level is not an enjoyable expierence.
That about covers wildlife and riding style, but narrow curvy roads have another threat as well. Riders who have partied too hard and are riding impaired in one way or another are a hazzard. Don't be surprised if you encounter another bike over the centerline in your lane on a curve or passsing a car when they clearly should not. Coming around a curve in the Custer area it is not uncommon to have a 2000 pound bison standing in the middle of the road.
A threat that I think is pretty much new this year is that vacationers from as far away as Florida are renting RVs and coming here in droves. None of them have ever driven a large vehicle and don't seem to know where their right side is in relation to the roadside. I have encountered them multiple times so far this year already. Usually both rear wheels on the left side are completely over the centerline on curves. That puts a two to three foot wide obstacle in your lane.
I am an ambulance board member for a town in the central hills, so I could give you some really ugly stories, but my intention is not to scare the hell out of you. If you ride safe you will find that the Black hills is some of the best riding in the US. So, have fun, but stay safe cause I will hear about it if you don't...
When in towns, be sure to stop by fire and ambulance services and say hello. These are the folks that will likely save your life if you need their help. They saved mine.
JB