Changing the drivetrain

Does anybody know of anyone who's converted a FWD car to a RWD?
If so, how much did it cost (roughly)?

I would imagine that a tunnel would have to be made that protrudes into the cabin in order to make clearance for the new driveshaft (among many, many other things of course).

How will this affect spoon engine installations?

Thanks.

It can be done, but it's expensive and difficult.
The engine is mostly mounted diffrent in a FWD car than in a RWD, this can be changed but you will need new mounts (maybe you must adapt the whole enginebay for the new engine setup), and a new gearbox. You must also remove the driveshafts to the front wheels and change many of the parts in and around the front hubs (maybe new hubs and suspension).
Then you must have a rear diff and driveshafts, and of course a driveshaft to the rear wheels. You must probably change the most of the rear suspesion. A bigger tunnel for the driveshaft can also be needed.

Then I think it's smarter to convert to 4WD. Gearbox and diffrentials you can use from an Audi S2, or if you want a little better system you can buy Prodrive gearbox and diffs.

What car are you considering doing this on? How much sense it makes and how much it would potentially cost depends completely upon the application.