So i've cut out the rust and refab'd the rusted part of the pan. Should I do more than prime and paint the pan before puting the bed floor back over it?

I would spray some weld through primer on it before you weld in the to part. Then use some seam sealer on the underside to seal up the repaired area.

An observation....I'm not sure if all the 80's K5's are the same but when I cut the tailgate post in the void for the tail light wires there was a large amount of what feels like carpet pad material. Probably holds moisture and helps rust.

ooohoooh another quickie....do welds usually rust through? There is a factory spot weld that I have ground half way through the steel and the weld still show rust.

I do not remember anything being inside the tailgate post on my K5 when I was under there poking around. It could just be sound deadener or something a mouse pulled up in there to make a nice home...

As far as factory spot welds rusting through... Typically it is the surrounding metal that rusts away, leaving the actual spot weld...

You think we should combine this with the Floor Panel Replacement thread....seeing as they're all part of the same project?

Probably. Just leave this one alone and use the other one.