Cooling issue on highway

First thing I'd be trying is taking off the fan and shroud completely then quickly jumping on the freeway and seeing if the problem still exists. If it runs cooler, bam there's your issue.

If it turns out to be the fan/shroud... I'd dump that shroud entirely and just mount the fan basically up against the radiator core. Or get a fan and shroud out of a Charger/300 that came with the 6.4L.

Does seem like your cruising ignition advance is pretty low though. I know those engines don't need much timing from the efficient chambers and dual spark plugs but below 40* at cruise is nothing. Probably getting extra heat rejection into the engine from the late burn.

Food for thought, my Duster runs about 185* around town but on the freeway it gets up closer to 195*. Has a high-flow 195* t-stat, high-flow Milodon water pump, ECP aluminum radiator and 7-blade mechanical fan with viscous clutch drive and factory shroud. But it's also a carbed 360 with 9:1 compression, quite different than a EFI 6.4L Hemi with 11.5:1 compression so not a great comparison tbh.