My 1964 impala ss 327 runs 200 degrees on hwy and 220- 230 in slow traffic It has good 2 core radiator water pump is new Should you try a 4 core desert cooler?
If you haven't tried doing so already, you might try changing
the thermostat. I had the same problem with my own 64 Impala but
didn't discover the real cause until I blew a head gasket and had
to yank the motor. The water jacket in the engine block had about 3
inches of sediment in the bottom of the block and was so dense it
had to be picked out with a long metal rod. It was coming out in
chunks. A new water pump, thermostat, belts and hoses made no
difference. Not trying to scare you, but you can find out by
removing the water pump and use a long metal rod to scrape/poke the
bottom of the water jacket. You'll know how deep the sediment is
anyway. Be careful not to distrurb it too much if you plan on
driving the car. The loose crud will get stuck in your heater core.
Carl King http://www.classiccarauto.com