Cooling issues, could use some advice

The fact that it runs cooler at speed tells me there's not enough air or not enough water flow or both at low speed.

I'm not familiar with magnum / LA all my stuff "stays LA" You don't have some mismatch with magnum timing cover / LA pump or vice versa?

Any chance the block is effed up, IE dirty or epoxy filled?

Have you checked (hold at high med, med high, and high RPM) that the bottom hose is not pulling shut?

I don't know how many stats you've tired, I would try another.

Have you tried just pulling the hood clear off on a hot day to see what happens?

My gut thinks frontal grill opening is not the problem

It might be the fan is not doing as well as you think.

I don't think the heater hoses are the trouble, but you could throw a marble or something in the hose to plug the flow.

Too bad you are having trouble. That's a cool concept.