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Link here. The article gives an excellent and detailed overview of where construction of SLS presently sits, what the problems are that still remain, and what they have done to overcome them.
What struck me most however in reading the article was how long it takes them to do anything. For example, it appears they will assemble the rocket in the spring of 2018 for a December launch. In fact, the description of this assembly in this article partly explains to me why SLS will have the ungodly slow launch rate of at best once every two years.
In addition, the article describes how NASA has handled a number of engineering issues that have come up, and for each the approach seems to me to be more complicated than necessary. However, I am not an engineer and have never been part of this kind of work. Maybe everyone does it this way.