Fuel tank capacity and door sill cracks

[TTM]

Hello,
   
   While looking at an original Mk2 Cobra recently I noticed some light cracks on the bodywork about halfway up its door sills.
   
   Someone I believe to be in the know mentioned this was a design weakness that would often appear "after a while" if always filling up the fuel tank to its maximum capacity.
   
   Thankfully my Ace does not suffer from this issue but I have been wondering if other folks on here had heard about this before and accordingly avoided filling up their fuel tank to the full?
   
   Thanks.