A Detroit might sound amazing but I can't help feeling would be completely the wrong engine in a Martian. It seems to me the issue with the B81 is not the power per se - after all, most contemporary large truck diesels were around that power output - but the comparative lack of torque. I'd have thought the Detroit would suffer a similar issue in that although more torque is there, my experience is that it's in a fairly narrow band and you have to rev the thing to keep it pulling.
With a vehicle like a Martian I'd have thought you'd want something with a wide, high torque curve which will pull well from very low revs. That might give you a gearing problem given that sort of engine tends to be fairly low maximum governed speed but there are other ways around that - a gearbox with a very long overdriven top gear(s), for example.
If the Cummins is a C series IIRC they're around 190 - 280 bhp, 600 - 800+ lb ft torque and about a 2500 - 2700 rpm governed speed depending on spec - that doesn't seem too shabby to me and certainly better than 220 bhp and 335 lb ft for a B81 though the peak rpm is low.