radiator??

Weird that is will idle fine. But only when moving, it will get hot. Usually that's radiator clogged or too small.

What is the exact radiator you have now? How many rows?

Have you tried an 17-18" factory viscous fan? Those blades are much wider than a flex fan.

I have a 421 stroker with about 450 HP and cam close to yours: Comp XS274S . I get 15-20 degrees from top to bottom of radiator.

Could you old motor have dumped rust/debris in your radiator and clogged certain portions?

Have you heat gunned different sections of your radiator core to see if maybe a certain section is clogged?

When I did my motor I hand picked the coolant passages clean of rust build up and debris. And water+shop air gunned the passages. I can't see how modern hot tanking could have chipped off that rust scale.