I think the confusion is coming from the difference between 1G and 2G. The 1G has a totally different floorpan between FWD and AWD and a swap is pretty much impossible because in addition to all the drivetrain parts you'd need to cut out the old floorpan and weld in the new one.
Now, with the 2G my understanding is that the basic floorpan is the same. So you drill some holes, weld a driveshaft bracket or two in place, maybe change the crossmember up front, and install the dif and suspension. It can be done relatively easy.
IIRC RRE did a 1G swap for like $7000 and said never again but have done a half dozen or so 2G swaps on spyders mostly.
With all that said a Turbo AWD Spyder would be supa-pimpin' :pimp:
Luke... err, I mean, Andreas, the force is with you