I have now broken(yes broken) the exhaust valve rocker arm and bent the push rod on my 2003 F-250 twice on the #1 cylinder. I tow a large horse trailer (18,000 GCVW) all year long. I only have 89,000 miles on this engine. My questions are:

1) why does this keep happening? Is it an inherent defect in the engine?
2) Is there an easier fix than replacing the whole engine?
3) what can I do to prevent this from happening again?
4) if I have to replace the engine, is there a possible to put a 7.3 liter in its place?