If your jackstands are under the main chassis (not the subframe), it would suggest to me that your subframe mounts are allowing the subframe to droop too much when it's unloaded. You can see the mounts between the subframe and main chassis with the front wheels off. You should be able to compare the mounts in the unloaded vs loaded condition by inspecting them first while sitting on jack stands under the main chassis, and then jacking under the subframe until the jack is carrying the weight of the car.
Also depends a bit on how much fan clearance you have normally. If it's minimal, then a slight droop may be all it takes.
George Davis
'69 280 SL Euro manual