[KiwiBacon]

There are a couple of things to be wary of.

It sounds like you've got a few good ideas already (overspeed and all that). The way to know for sure is to identify your cummins turbo. It's probably a Holsett, sounds like earlier ones used the H1C series, later used HY or HX or whatever they're called.
Once you've found the relevant compressor map, plot out your maximum engine airflow on it and see if it fits.
If the max airflow and boost are still on the map then you won't overspeed the turbo.

Diesel turbos typically have a smaller exhaust housing than the equivalent airflow petrol engine turbo. This is because diesels put out colder exhaust. This will make the turbo spool up earlier than usual, but it will also be a restriction at higher rpm and flow. I know you can get a range of exhaust housings for those turbos, but I don't konw the details. Worth looking into.

If you need help calculating airflow, just ask. You need to know engine size, rpm, any intercooling and intended boost.