Running Hot on the Highway

My car runs hot on the highways, around 200-210 or so degrees driving a steady 65 mph. Before someone says "that is not hot" I don't like it and don't want it running that hot. When I get off the highway it slowly creeps back down to 180 - 185 or so. It was 85 degrees outside that day. It can idle all day long and never get hot no matter how warm it is outside. Never runs hot when driving around on the back roads. Here is the set up:

Flowkooler water pump (new).
Original crank and water pump pulleys for non-AC car.
Mr Gasket 160 high flow thermostat (new)
Mopar Performance viscous fan package (new)
3:91 gears
414 small block stroker w/Edelbrock heads, 500 miles
TTI headers and TTI 2 1/2" exhaust
Original 340 w/AC radiator with NEW 3 row core
Reproduction fan shroud
Spring in lower hose (spring is 4" too short)
727 trans with 2500 stall.
50/50 mix anti freeze and distilled water.
8 pound radiator cap (recommended by radiator shop).

Wondering if that spring being 4" too short on the lower hose is allowing it to get sucked shut at high RPM? I can pinch the hose shut with my fingers no problem but I can't find a longer spring. Is the radiator not up to the task? I would think a new three row core would do and it only runs hot on the highway cruising at high speeds.

Any suggestions greatly appreciated. Like I said above, I don't want it running that hot on the highway and want to find a solution.