The 990 SSF doesn't use a standard mATX mobo. It's more like DTX, but wayyy wider. You'd have to use mITX.
Keep in mind there's only a single 80mm intake fan in the case. You'll need a new, low-profile CPU cooler (the stock dell one won't fit a standard CPU socket placement) which will be downdraft. This means you'll have to keep a pretty low CPU power output.
Dell typically uses some sort of proprietary front panel power/USB connector to attach to the mobo. I cant make out a mobo header that would supply this functionality on the pic below except for the header next to the rear panel audio port cluster.
Usually you'd be able to...