Check out Ray's project journal on the first page:
I did a similar thing. Here's a couple pix from my build page:
I've still got to update this page because I've since finished the frame rails and I could take more detailed pix. My driver side rear trailing arm perch and corresponding unibody frame was rotted. For my frame rails I chopped out a channel right through them and ran the rails right to the trailing arm, creating a new perch for them.
I 'm pretty sure Ray left his alone. He went underneath and welded tabs up in to where the trailing arm mounts to share load through the frame rail.
The last place to get a different POV is Reid's garage page:
http://forums.h-body.org/garage.php?mod ... cle&CID=12
He built a weld in frame that didn't modify any existing structures. He's got plans for them too. I went a different direction but the plans helped me plan my solution better. It's worth taking a look at...