Jesse,
I don't think any combination of the 22R belhousing will work because of starter and ring gear alignment issues, as well as starter placement. The spacing would be off for the input shafte as well so something would have to be customized to make it work.
Is there a reason you're trying to go with a W58 instead of the J160 that came with the IS200s?
I've seen some pretty healthy 20V 7AGs dyno at about 180whp/180wtq with a decent tune and very mild cams. But if you're deciding between a Beams 3SGE and a 20V 7AG, the Beams is such an awesome motor with the dual VVTI, but you're going to have a hell of a time finding parts and making it work in a lot of cars.