It's a pretty common problem with early cars like your Chevy and others like 39-40 Fords. The shape of the front end sometimes doesn't cause air to go through the radiator, but it gets deflected around the front instead. Another problem can be that the air in the engine compartment has no way to exit. When you put a big V8 in an engine room that originally held a smaller 6 or flathead 8 it tends to block some of the airflow in and out of the engine room.

Puller fans are much more efficient than pushers, and a shroud is a must so it directs the air clear across the entire core. A high quality fan is also much more efficient than the Autozone types or even the ones sold by most places like Summit. I like SPAL and I have seen good reports on a Lincoln Mark 8 (I think) fan setup.

For a cooling system to be efficient all the components have to work together. You need good airflow both when sitting and when moving, and good water flow also. Your AC unit is not helping, but lots of guys are running air and having no overheat issues.

You mention two 12 inch fans. Believe it or not, one 16 inch of good quality will out perform those two fans. And I stress again, make up a metal shroud so the core is sealed and the air has to be pulled clear across the face.

Don