It probably wouldn't fit in the case quite right. BTX is actually longer than an ATX on one side, essentially like TFX, though it's not truly a standard as far as PSUs go; all of these are some kind of Small Form Factor, but they do vary dimensionally. Also, I think the screw layouts vary quite a bit. Even getting a PSU to be held in my two of four screws could be a chore and may require case modding. Ultimately, measurements, or even good guestimates, by Trainzfourever as to what he's got would be needed in order to give any suggestions.
Then again, maybe he's handy with tools or doesn't mind a power supply dangling out the back, lol.