I just found this thread. Here's my two cents... The radiator can be ALMOST flat, but needs just a little tilt (as little as 1/2 inch) so that air can rise to the highest point to be bled off (Obviously, your bleeder has to be at the highest point of the radiator). If it's completely flat, air bubbles can get trapped in various places. If there's air pockets in there, then the coolant isn't touching the metal. The heat is removed from the coolant by transferring to the metal and then being blown off into the ambient air, so air bubbles inside the system are your enemy.

Back in my stock car racing days we used to use the radiators out of Olds Cutlass Diesels because they had 4-row cores and we thought they were the best thing available. We used a restrictor washer instead of a thermostat. On a really hot night the car would run hot anyway, sometimes 230 degrees. Finally, one time when we had a few extra bucks, we bought a Howe aluminum radiator. We had to put a thermostat in it because we couldn't get it to run hot enough!

If you use a 19 x 31 Howe or Afco radiator, it should cool OK with the stock pump. Consider the Pontiac Fiero...