I had a similar high speed cooling problem as I have a 400 convertible with air conditioning. It turns out that my car was missing the lower air baffle valence like Bjorn mentions above. It lays flat under the car and connects to the front valence routing air into the radiator. Without this, it's easy to see how air would pass under the car vs. thru the radiator. My understanding is that only original 400 cars or air conditioned cars came with these. I also think that many could have been left off by repair shops, etc.
I bought a fiberglass version from Ames ($99) and installed it. The fit was good along the front but the two tabs in the back were very mis-aligned and required some serious work to attach. The car does run cooler at highway speeds now. Does your car have this? It's flat with a 90 degree bend in it. I couldn't find an original so I had to get the fiberglass version.
The previous owner of my bird obviously did a lot of things to try to cool the car. These included adding an electric fan, eliminating the heater core from the cooling loop and removing the thermostat. I changed all of these things back to original but nothing really helped until I added the valence.
