I went through this with my 91 K5. When I asked the seller about his 84 K2500 the first question, "any water leaks". None was his answer.
So here we are. Dash pad out, rug and seat out. Cowl off.
Looks like the windshield is leaking (big surprise ) and both cowl vents. Non AC truck mind you. Now apparently there was a TSB for the cowl leak. You remove the vents and add water deflectors. The number is listed as unavailable so I may be up the creek on that one.
Feel free to chime in with your water leak storys.