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Dowdy washer :). It's like a washer with a rubber seal bonded to it.

you would use an annealed copper washer wouldn't you.
like a squash washer, or heaps of Teflon tape until you get a proper washer

Also what a lot of people don't realize is the air is drawn through the radiator by pressure differential. Any fan engine or electric cannot compete airflow wise once the vehicle surpasses about...

I would stick as close to OE as possible Winnie just fitted a HD radiator to his 4.2 and seems very happy with it.
I'd buy a new HD radiator and reco your clutch /thermo fan and go from there ,check...