Actually, according to Commodore owners in Australia, installing an LS1 is a very easy swap as a version of the Commodore comes with the LS1 from the factory.
And, making the Northstar engine work in a RWD setup isn't that hard at all.*
http://www.chrfab.com has made a business of selling Northstar engines setup for sandrunners or even hotrods. I've seen the Northstar in hotrods back at car shows in Tennessee. The owners never said it was a hard swap. Now, motormounts may be an issue, but I expected this to start with. But, I know the Catera (which is a Holden and Opel underneath after doing research) will accomodate the Northstar.
According to the above website, they use an S10 manual transmission but I was thinking that a Vette 6-speed would be stronger and better. GM people I have corresponded with online all have told me the Vette mission will bolt on. Even if they are mistaken, the S10 mission will work fine. You can find Northstar engines for sale on EBAY fairly cheap these days (often under $2500). A used 1997-98 Catera can be found for under $6000. I know shops back home that I can trust to do a good job, so for $10,000, I think a V8 powered Catera would make a damn fine car.
*Its been done here in Japan and seems to be getting more popular due to the weight savings. Swap an MR2 mid-engine configuration into a MarkIV Supra which is a RWD configuration. 500hp+ and a much much smaller engine that sits behind the front axle and is lighter than the standard MarkIV inline 6 engine.
I'm not saying a Northstar in the Catera will be a cakewalk, but in all my research, I see no reason why its either impossible or cost prohibitive either. Since the Northstar will fit into streetrods and work RWD, there is no reason to think it can't in a Catera. It will be awhile, but I can't wait to find out. I would love to see someone else do this swap even if I'm not the first.