At the risk of outing myself as something other than a mere diesel enthusiast......
I cannot in good conscious recommend any V config Cummins as a suitable replacement for anything Caterpillar in the maritime world. Inline config, maybe, depending on the Cat. Electronic engines are exciting on paper, but can become a horrific nightmare out on the water, expecially common rail fuel systems...
If you have a good running but tired 3400/3500 series Caterpillar, short of an non-repairable block failure, or a boat that requires USCG Polar Sea/Star level disassembly to R&R, rebuild it. Or replace it with a like for like.
A 3126 can be hit or miss but if they're not HEUI or common rail, there isn't much there to break if you watch your temperatures. The Cummins inline 6 non common rail or electronic pump can be a viable alternative, but it would be just as financially feasible to rebuild the 3126 as swap it.
Dealers just love to sing the praises of electronic engines and common rail computerized magic voodoo fuel systems, and sales account managers(harks) just love to PowerPoint your fuel savings and cost bottom lines with fancy animation and wonderful oooooh's and aaaaah's but the guys in the white coveralls waiting at the dock for Crowley to tow you home will tell ya, stay mechanical until you have absolutely no other choice.