I have this leaky problem with the 1982. But there is definitely more damage on my car due to rust. The two bottom corners lack some metal seat behind the polyurethane (or butyl) caulking. I would say about 2 inch from little I can see from under the dash.

For now I try various solutions that do not seem very sucessful; like Leak Sealer.

It seems the best solution is following your receipe with wire wheel, POR-15 metal ready and POR-15.

Except I figure I still have to re-construct this metal seat lip so the seal rest on solid ground.

Question: That would mean some dash removal and TIG welding work I am afraid ?

Car has 440k km (270k miles), is visually in fair to good condition, need a paint job although it has no rust through, no hole in floors yet, does not burn any oil.

Winshield is due for replacement due to lack of transparency when facing sun.