I think you guys are making it a little too complicated. I see the op live in CA and they do some wierd things in that state. However, in AZ it is simply not that complicated, from a legal standpoint. I have a 1978 k10 that is a rust bucket and I took a very clean 1978 c10 and put all of the metal, cab included, on the 4wd frame. I own both and hold titles to both. When my project is completed in a few weeks, I will simply own a 1978 c10 on paper, that happens to be a 4wd. It is perfectly legal and I am currently in a local law enforcement traffic unit and have been in LE for 16 years.
As for the technical side of the op question, you guys are right on, as usual. I cut the hump out of the k10 and put it in the c10. I will post pics soon of my truck in a build thread as my daughter was taking pics this weekend as we built the motor.