I have the exact same problem with the standard steel oil pan in my standard 1959 750D. The undamaged cross member likewise interferes with the oil drain plug. The mechanics must have had a way around this in the day. I make do with removing that cross member at each oil change. I have not found the right offset wrench to remove the drain plug without removing the cross member. I'd love to know where to find one.
The perfect is the enemy of the good. Voltaire