I'm not so sure. I have 2 '73 K5's, one I used for parts for my project, the other one....anyhow, in tearing down both trucks the crossmembers were different.
One was TH350/NP203 and other SM465/NP205. The memebers were in the same location fore-aft, but the mounting holes on them were drastically different. The NP205 version also has a side bracket that bolts to the frame rail and helps control torque under power