I'd be pretty leery of this swap. These motors are notorious for having the head bolts strip out of the block. My STS is sitting in my shop right now with this problem. It's a nice motor, but wide, parts aren't cheap, not many performance parts, and you can't get undersize bearings for them.
I really wish I could forget about fixing mine and just put the time and money into my 280/LS1, but I need my carpool car. Good luck with whatever you choose to do, there's alot of Northstar info at cadillacforums.com