The "599" block casting (last three digits of the block casting number) is/was popular, as it was the first casting offered for the newly introduced 6.5L diesel engine for the 1992 model year. This casting first appeared in the 1991 model year as a 6.2L diesel, and that same casting was used till the 1995 or so model year. In 1992 and 1993, truck buyers could get either a naturally aspirated 6.2L diesel or a 6.5L turbodiesel. Both used the same block casting. The only difference was cylinder bore diameter.

Thousands of 6.2L 599 castings have been bored to the 6.5 specs - thousands. And, I recommend it. But like others here have suggested, I'd not bore any other 6.2L block castings to a 6.5.

And, the 599 was also one of the most crack free blocks GM produced.

Jim