Radiator upgrade experiences?

Don't get wrapped up in 3 core or 4 core. You want to concentrate on tube size.

A 3 or 4 core rad might have 1.5" or 1.75" tubes. You can get a 2 core rad with 2"-2.25” tubes. This equals more surface area for cooling.

And like 72bluNblu says "how hot is it actually running"? 200-210 degrees is not too hot for a big block.

Just for reference, I ran a 2 core rad with 2.25” tubes in my 1970 Firebird with a built 455 and it hovered between 190-200 all day long.