His wifes additional weight percentagewise overall is of little consequence in my opinion. What may have aggrivated this problem could be pushing the car harder than the last owner did or just luck of the draw.
Good old coincidence in action. The 3.5 can really accelerate if you come onto it. Something to see in a good example. Just get it moving a little first. The head cracking with car not showing any signs of overheating is a possibility. If no overheating observed the head gasket gains in favour. These are just my opinions though. Others will and should vary. Please post whatever it is you find out. Again most the 3.5s give a clear signal the rod is bending or has bent over time.