Your description may be a bit confusing to some, since there are several different sections where 87 and 16A run close by. But I assume you refer to the section of 87 exiting Custer SP to the south toward hwy 385 and Hot Springs SD.
I don't recall seeing any official restrictions. This section is a narrow 2 lane road, with no shoulder in most places, lots of twists & turns, lots of up and down. But its a beautiful, scenic drive as long as you can handle the sharp curves. There are official campgrounds along this section, so its passable by RV's of normal sizes.
The northern section of 87 (Needles Highway) is the one with 2 tunnels with only about 12 ft vertical clearance.
Jim