will it fit perfectly with no problem at all? or do i need a bunch of cutin weldin and custom engine mounts? thanks

Common sence will tell you since it never came in the car, it will take lots of custom work. Of coarse it fits in the engine bay, since the MKIII also came with a 3.0 I6, but it is obviously not plug-n-play. Also maybe you didn't see the exact same thread two posts down...
http://www.automotiveforums.com/vbulletin/showthread.php?t=237720