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In this paper, we study a fundamental and crucial problem of building timing-driven over-the-block rectilinear Steiner tree (TOB-RST) with pre-buffering and slew constraints. We pre-characterize the tree topology and buffer distribution to provide accurate timing information for our final RST construction. In most previous work, the routing resources over the IP blocks were simply treated as routing blockages. Our TOB-RST could reclaim the ``wasted'' over-the-block routing resources while meeting user-specified timing (slack and slew) constraints. Before fixing topology, a topology-tuning is performed based on location of buffers to improve timing without increasing buffering cost. Experiments demonstrate that TOB-RST can significantly improve the worst negative slack (WNS) with even less buffering and wirelength compared with other slack-driven obstacle-avoiding rectilinear Steiner tree (SD-OARST) algorithms.