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Here is a quote out of the service manual for replacing this belt
CAUTION: When aligning crankshaft and camshaft timing marks always rotate engine from crankshaft. Camshaft should not be rotated after timing belt is removed. Damage to valve components may occur. Always align timing marks before removing timing belt.
While the manual doesn't specifically state that it is an interference belt this indicates that it is.
The service interval on this is 120K. While replacement of the tensioner is not necessary for the job we have found that there is a high failure rate on them before the next service interval. In other words the tensioner is likely to fail and take out the timing belt before it is due for replacement again. We always recommend replacement of the tensioner, but leave the decission up to the customer. In most cases they take our advice.
At this point there is no way to really know with certainty that you had head damage. if you did you got a very fair price. If the truck has a new head on it I would think that you really needed it for the simple reason that there are lot easier ways to rip a customer off than to do all that work.
I hope that this has helped you.