In northern BC presently with my boat, arriving in Ketchikan AK in two days. As GDavid mentioned, I removed 871 Detroits replacing them with 210 Cummins in my 1971 45' Hatt convertible. I did this nine years ago and it has turned out great for my application.
A couple very strong provisos, though. Doing this will cost you a lot of money that you will never get back on resale. Never. The only way it makes sense to repower a boat that old is if the boat is a long-term hold, maybe your last boat. In that case you will recover the cost through the use of the boat and the utility it gives you. For me, up and down to SE AK with low-time engines and gears is worth a lot. I did a lot of the work myself on the re-power but the hard costs, recon engines and gears, started me at $100K. But it's so much more than that. Motor mounts, exhaust run, wiring, countless changes. And I had to re-ballast the boat. A bobtail 871 weighs 3000 lbs, roughly. A 6bt 5.9 Cummins weighs 1000. I took out two tons of weight way down low and the boat became very tender. I talked to Hatteras and put the weight back in with their advise on placement and the boat is now true on its lines and feels stout.
I can empathize with you wanting to do this with an old Hatt. The '70's Hargrave designs are the best of the bunch to me. It always rings my chimes to look at my boat's shearline and profile. But give yourself a reality check before you embark on this. Lots of money involved that you'll never see again. You have to REALLY like the boat to do this.