Same here, for the same equipement. Did not use heatgun but oven, wrapped in tinfoil. Same results as you reported. Finally sourced a compatible GPU from eBay,

Same here, for the same equipement. Did not use heatgun but oven, wrapped in tinfoil. Same results as you reported. Finally sourced a compatible GPU from eBay, an ATI 5xxx from the next model year iMac. It was much much cheaper than sourcing an ATI 4xxx. Opening the iMac the first time was very stressful.