I bought a 73 450SL about 8 Month ago and have been battling to locate what is causing the exhaust manifolds to glow red hot after going down the hwy at 3000 RPMS for 5 minutes, pull over to the side of the road in the dark and see the exhaust manifolds glow where they connect to the exhaust pipe.

What have I done so far:
-Replaced the exhaust system with factory original
-This car doesn't have cat. converters
-Changed out points to electronic ignition, plug wires plugs and new coil
-Replaced the water pump/thermostat and tested flow rate of radiator.(was running hotter than normal)
-Changed out the computer circuit card for the Fuel injection.
-Changed out the Manifold Pressure Sensor -Adjusted fuel pressure
-Checked timing phycial marks and with a light to spec. 5ATDC (I can advance it up to 20BTDC and it decreases the red hot glow the closer I get to 20BTDC The car runs smooth but exhaust manifolds overheat!! This is a tough one, no prior history on the car. Any suggestions? Thanks in advance!