I've always wanted to drop a 540 from an XLV into an SS440, but never have the time or money. I love the SS440, one of my all time favorite sleds!
The XLV is basically the same sled as SS440, except the engine and the 2-up seat.
If you buy a 540-engine and then sell your SS440-engine, the cost should not be that big...
The 540-engines from Excel-V, XLV, Viking (VK) or a SRV, they all fits right in to the SS.
You might have to use the front motor mounts from the SS. And you should use the clutch setup (weight & spring) for the engine you choose.