Hi Everyone. Jim, this is a popular question and the search feature will turn up a lot of information.
But, to get you started... Anything that bolts to a block, is easily changed, and you should assume has already been changed (maybe even more for a commercial vehicle). So the heads aren't going to help your question get answered.
The configuration of the engine block is the only way to identify the year of the engine. There is a very informative thread around here somewhere with a ton of photos and simple explanations.
Passing the question of where that thread is, to the next member in line.