I used a 300 shroud on my 425. I turned it upside down so the side-to-side offset fit the GS radiator tanks. Then cut back the fan opening so it fit between the rad and the fan. A flying blade off of my flex fan destroyed that shroud.
Next up, I used a clutch fan from the junkyard with no shroud. It works well. Clutch was from a 1980's V6 Regal, and the small diameter fan I got off of a Cadillac. I believe it's 18 inch diameter.
On my Red '66, I'm using a Griffin rad (universal fit, about $200), and a old GM 7 blade fan. That one runs cool as well.