Tim, have you actually witnessed where the water enters,is it simply overwhelming the seal or is it coming through a seam?
As you will be aware,the cowl section is not a one piece pressing.Paint alone will not seal it.
I imagine they were probably seam sealed all those years ago,but the sealer dried,cracked and fell out over time.
Sounds like you need to get flexable and get under the dash with a torch while Mrs Radial,or some other helper sprays the truck with a hose.
