The plastic retainers are the problem, as they have hardened and do not flex as they did 45 years ago. As Larry wrote, the grills are made of very soft metal, they bend (and crack) easily. The advice to push upward on the pins from below is good advice … as long as you don't bend a pin. I damaged one of the grills on 6018 during removal a decade ago. I was carefully (slowly) pulling up from above.
Perhaps using some spray lubricant on the pins prior to trying to remove the grills may help.
If the car is going to be repainted, then perhaps the application of some heat would soften the plastic retainers.
Also be aware that the '74 grills are different than the grills for all other years. 1974 Panteras park the windshield wipers on the opposite side of the window than all other years.