Thanks for your replies.
As far as I can tell, the passanger side seems to be ok (see below). It looks like the PO used decent parts for the conversion, but (unfortunately) he cut some corners. Not only with the power steering...
My definition of safe modification and proper restoration/fabrication is clearly different.
No can do. The crossmember has some damage (third pic), but it doesn't seem to be too bad. Luckily, GM used pretty thick steel.
Captainfab's adapter looks interesting. How can I tell if it fits the steering box on my truck? I'm kind of worried drilling even more holes through the frame, though. Mine isn't nearly as clean (see previous post) as the one shown on the fourth photo...