You mention the heat gain only at idle. That narrows it down to either air flow or coolant flow. There have been several good suggestions given here about air flow and you also seem to have a good knowledge of it. As far as coolant flow you may want to try a smaller diameter water pump pulley which will run the pump faster relative to crankshaft speed and create more flow. A second benefit is it will also run the mechanical fan faster. It doesn't take much, as little as 1/2 to 3/4 inch smaller, to make a difference. A little digging around checking out the small block Chevy V8 pulleys should find one. Just remember there is a short and a long water pump on those V engines depending on the year and the pulley hubs have different bolt patterns and overall the pulleys have different lengths that can cause belt misalignment. A slightly longer pulley can be spaced out correctly with a pulley shim kit.
If you have a new parts store water pump on your engine the hub flange may be drilled for both pulley bolt patterns. Most of them are. That would make it easier to come up with a pulley in that regard.
I hope this info is of some use. Running hot is no fun.
