I assume that this is for a mid-tower GX-280. There is only one trick that you need to do to get this BFG supply to fit in the 280. You will need to take out the four small screws holding the top of the PSU. Then pull the wires off the on-off switch. Then take pliers and gently squeeze the sides in so that the on-off switch can be pushed out of the PSU bracket. Then make sure that the switch is in the on position and reconnect the wires to the on-off switch. Put the switch back inside the PSU. Screw the lid back on.
Now the PSU will fit into the GX-280. The back of the GX280 is not open like ATX cases, so the supply has to only have a power receptacle in the correct corner of the PSU for it to fit the GX-280.
Now, I am doing this with a BFG 550W supply. I am running a GTX260. What are you powering with 1000W?