DD 6-71N 1977 question

Yep, in line Detroit Diesel. Pic of valve cover could cover more details.
So many people have marinerized the Detroit blocks sometimes they are hard to recognize. Remember what Chris Craft did to SBCs in the 50 & 60s?

I'd dig in to who will support the different parts like the heat exchangers and manifolds before I got serious.
You can't invest your time or money in the machine shop building your parts for service needs.

The engine pictured is typical of the time for after market, higher hp 671. Later Detroit quit selling engines to after market builders because it wanted the market for themselves. There are a bunch of companies that make replacement heat exchangers for most engines. Also many DDs in commercial use are being replaced with tier compliant engines. To get the tax break the block has to be drilled to make it useless, but all the parts are good. There are people stripping those engines and selling the parts - many on ebay. It's not hard to find a replacement anything for a 671. Even a really old 671. Also there are foundries that sell new marine exhaust manifolds. As I remember $300 for a 671 marine manifold, just like the original. Also in aluminum.

That's not that old of a TollyCraft. I think after DDC took more control of their marine engines. Uh?
It's not an engine problem. Many of the parts are still interchangeable (2 valve heads) since they started (pre WWII).

If the HP was increased after market then the cast iron heat exchangers cooling capacity was marginal.
Lenco makes a whole variety of exchangers similar to what is seen in the pictures.
I don't think that would be a deal breaker.
lencocoolers dot com

On the other hand. not a lot of tight garbage bolted to the front of the engine.
N/A air silencer, simple block vent, probably a gear drive alternator bolted on the back with the raw water pump.

In looking back, I kind of like it.
I don't like following up behind somebody else re-engineering.

With the serial numbers, you can get the delivery dress and what was ordered from DD.

I can't imagine a 671 NATURAL ever pressing the stock heat exchangers ability to cool any condition as long as water is flowing.

In the deep background and sides of the pictures down there, The engine room is a mess.
Messy E Rs always confuse me. Most of your investment is down there, it should be as clean as your galley.
I've walked off boats and blow away schedule a few messes before.

If it is really a mess down there, insulation hanging, junk stacked/piled, dirty, dimple lit (just from a few pictures), along with non factory mods to the engine, I would greatly re-consider my purchase offer.

J&T was getting 310hp out of 6/71 naturals. I have a pair. If the seawater temps are high the original exchanger can get marginal. That looks like a Lenco exchanger, much more efficient than the original. As was mentioned above, Detroit parts are plentiful and I might add, cheap.

300hp from the 4 valve head. Yep, it is still a great simple engine. Life span still way up there.
We went there on an old 44 Striker re-power years (many) ago.
No temp issues but never got the props loaded enough before the owner sold the boat. Solid 170 degrees no mater how hard or long she was pushed against the pins.
She never slowed down going up the backs of swells. I miss that boat.

71s Rule