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The difference between a reading taken at full operating temperature and cold can easily be in the 20 to 40 pound range.
Compression for a hot engine is approximately 145 PSI for 7:1 heads and 160 to 170 PSI for 8:1 heads There are multiple head castings used on the engine over the years.
Any good automotive machine shop can resurface the rocker arm rocker surface.
Diesels can run at much lower compression, the main concern is that each of the cylinders are pretty much equal with one another. Check the cap for cracks or carbon tracks between the posts, wear on the center spring loaded post and deterioration of the side posts.