The X79 Classified uses the XL-ATX form factor (13.5 x 10.3 inch motherboard tray). There are 2 variants of XL-ATX the 9 slot and the 10+ slot. The 10+ slot variants allow for a dual slot video card to be used on the bottom slot of the board, as the card would hang past the board and need one more expansion slot at the back of the case, below the board to align with. On a 9 slot, you can run a single slot card in the bottom, sop a dual that can be converted to single via waterblock.
Below is a list and links for 9 slot and 10+ slot cases.
P280 (there are 4 variants in color and window options of the same case)
PC-D8000 (Also Support HPTX for SR2/X, and is double wide)
PC-V2120 (Available in 3 colors, also supports HPTX for SR2/X)
PC-A75 (Also supports HPTX for SR2/X)
PC-A76 (Also supports HPTX for SR2/X)