Looking at the probe for a second, I'm seeing major hood clearance issues. Finding a strong enough spot to put motor mounts would be tough, since they would have to be fairly low. Now, the LS1 fits in the corvette, so obviously it can be done, but it will depend on the particular V8 chosen.
Your next challenge would be routing the driveshaft. The focus has a huge exhaust tunnel that accomodates the driveshaft. If the bottom of the Probe looks anything like the bottom of the Contour or Cougar though, you may have a lot of sheetmetal fabrication ahead of you to accomodate the driveshaft and transmission.
If you look at a V8 focus, you'll also notice the exhaust almost touches the ground towards the front end. I can only imagine this being worse in a lower profiled car where the engine has to sit lower.
There's the gas tank too, it might have to be relocated. A relo can turn your Probe into a Pinto if placed incorrectly. Fuel cell in the trunk would probably be the way to go.
In a nutshell it can be done, but I think the driveability/inability to drive on normal pavement (potholes and speedbumps) would make it a bad idea. I'd look instead into a turbo setup for the engine that's in there. It would probably end up costing the same.