This. It's really not worth trying to get one off a shell as it'll get battered in the removal process.
As well as Offland's thread it's also described in my build thread and several others on the forum. In summary, you remove the centre console, handbrake and obviously gearbox & auto mechanism. Drill out the weld spots on the trans plate, chip away the underseal and then remove the auto trans plate. Next, fit the new plate (it WILL need some trimming), bolt or weld it back into place carefully alligning it, then seal it up. It's not as difficult as it may sound.
Edit, here's a link, it wasn't very well pictured, but should give an idea: http://www.mkivsupra.net/vbb/showthread.php?139393-My-new-Supra-rebuilding-Dude-s-drag-car&p=2011470&viewfull=1#post2011470