That's because the rockers were made of very thick gauge steel, the same as used on the frame rails. Also, the rockers had fairly good drainage / weep slots so that they usually wouldn't have water laying in them. Even though, many still did rust out. They are very expensive to properly replace.

 

Yea, the reason the floor pans rusted out was from leaky cowls. The inside of the cowl was never primered. It rusted through and allowed water to leak into the passenger compartment. You see, those cars were never meant to last longer than 10 years.

 

Mustang is a uni-body...there is no separate chasis.