Hi Steve,
The high speed heating is indeed unusual for these cars, but not unheard of. I have a few suggestions to check, starting with the least expensive first:
1) check fan belt tension. The belt is still providing torque to the water pump and generator/alternator and at speed it is the highest, if slipping the water pump may not be providing maximum flow.
2) reinstall a thermostat. The thermostat provides a back pressure to the cylinder head to lessen the chance of steam bubbles forming from the higher power needed at speed. Any bubbles formed and pushed into the water stream creates havoc in the radiator, reducing the ability to transfer heat to the fins.
3) since this is a rebuilt motor, did you retorque the cylinder head after a few heat cycles? It may be that you have a cylinder gasket leak that only shows itself at higher combustion chamber pressures. some shops provide a service to see if combustion byproducts are in the coolant. You may try removing the radiator cap and look for bubbles when the car is warm, may or may not show something.
4) i assume you have the stock air deflectors in place? you may also want to install a plate below the front of the radiator, between the frame rails. this will prevent the scavanging affect of the front shroud from pulling air down from in front of the radiator, increasing the pressure in front of the radiator. some make the plate into a scoop to bring fresh air up into the radiator providing additional pressure. i've tried this on my own car, and it does help.
5) you say you recored the radiator? what are the specifics on the recore? i had my radiator done by a local shop, and he installed a 'modern' 3 row, serpentine fin, 1 3/4 in. deep radiator core that was insufficient to the task and i had to have it redone to a 5 row, staggered tube, flat louvered fin that is deeper (2 7/8). It does exceptionally well at speed. Few seem to realize the options available to them when recoring a radiator, as i learned the hard way.
All this assumes, of course, that your gauge, carbs, and timing are spot on, and no brake dragging is happening.
let us know how its going?