That is correct. The minimal internal height you'd need is 52mm. That means 6mm standoffs and 0.25mm above the I/O shield. In terms of width, from the side of the shield to the PCIe-edge of the board, you've got 175.16mm, so at least 176mm internal width. Add 2mm to each of those, and that's the minimum. Depending on the design, you'd need up to another 2mm of height to clear the top of the I/O shield, and up to 2mm in width to clear the side of it. The bottom doesn't need special consideration, with 6mm distance from a 1mm thick sheet you already cleared the bend radius to the shield.