Unless you just want to see a lot more rolling grassland, I'd go back to the interstate and up to Rapid City and then to Custer.
In any case, take US 16 into the Black Hills and at the big interchange with 16A, stay right and follow 16 and NOT 16A and go to where it Tees into 385, then west to Hill City and then down to Custer.
Stop somewhere and pick up a Black Hills and Badlands Tourist map, its invaluable for getting around and virtually everyone has them to give out. I was depending on Google Maps and the first day on the way down to Rushmore from Spearfish, the Google Maps quit, they were doing a system upgrade at Google. Thankfully the roads have very good signage and I got a map at Rushmore.
FWIW and to blow my own horn a bit,
HERE is my trip report on my visit to the area last August.
Charles
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.