Any typical V8 bellhousing should work then. I'm not sure how big the front bearing retainer is on the 420's, if it's bigger than a typical Muncie or Saginaw then you may need to use a truck style bellhousing with a larger center hole. The transmission to bellhousing bolt pattern is the same as any typical 3 or 4 speed passenger car and truck trans.
I'm not familiar with the COEs so I'm just assuming that they have a provision for a trans mount on the 420. Most SM420s do not have a mount on them that I know of. I'm sure one could be fabricated to integrate with the rear bearing retainer housing though.
