In addition to the power (more below) the main reliability/maintenance issue is the sea water cooled aftercooler. If maintained regularly (disassembly, cleaning, new O-rings, reassembly and testing- see Sbmar.com) they can be operated reliably. If you do that maintenance and run them like your 210s they should last just as long.
But 420 hp is a lot for a 6 liter engine and if the PO ran them hard they might be rather worn out. If you want to run them hard, you run the risk of a seemingly small failure trashing the engine in addition to the wear. I would certainly add a Borel raw water alarm to give a bit more time to respond to a R/W cooling system failure at high loads.
Join the Sbmar Cummins forum and read all of the posts about this engine and its somewhat less stressed sister, the 370 to get a feel for the issues.
David
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