I don't think you have an ISL. Cummins ISL is a 9 liter diesel, I think you have a 500 HP ISM.
There used to be a 10-liter L-series Cummins engine. It peaked at 400 HP rating around 1991, then was stroked to become the 11-liter M. That thing has been boosted to 600 HP in pleasure marine applications...
Then the 8.3L C-series was stroked to become the 8.9L L-series.
Yeah, I'm sort of a Cummins nerd.