Overheating at idle

If it cools down when driving, then you don't have enough air flow through the radiator to cool it down or tune problem.
Base timing low, running lean, low fan/wp speed?
If engine is at 240, your fan clutch should be locked to have fan pull air across rad, you say radiator has no hot spots, at that temp it should burn your fingertips when trying to lightly touch to feel for cool spots.
Engine is modified so cooling system also is needed.
Confirm coolant Temp. What is outside coolant temp are we talking about?
Add a pusher fan that turns on at idle/slow speed for a test?