I could be wrong, but I don't see why you should have to change the bell housing. Remove the crash box, and install the SM420. The input shaft components should all be the same. You may have to shorten the driveline slightly because the SM420 is a little longer than the crash box.

Remember 9-11-01--God Bless the USA
JUSTICE, not REVENGE, will prevail

1951 Chevy 1/2-ton Pickup truck