Adding turbos

I would recommend getting the parts book diagrahm from D.D. for 671 TI's and changing all of the parts that are different. Or finding donor engines might be a lot easier and taking what you need off of them. A good DD dealer/mechanic might be the right way to go.

I'd probably look into changing out the reduction gears and props to achieve more speed as well. Our 56 Hatt had the mod done by a previous owner and we gained quite a bit over the stock boats.

Ouch
I'm not an expert, but I do know enough to get in trouble.
Some thoughts below;

Pistons, maybe liners, Injectors, inter-coolers, THE Turbo-charger, exhaust riser/adaptor, new exhaust dump can, Emgc air shutdown, air tubes, air filters, water tubes, maybe larger raw water pump, maybe larger oil cooler, maybe larger heat exchanger, prop pitch or new wheels.
Depending of what you have now and their safety rating; Maybe shafts, shaft logs, strut/cutlass. I'm assuming you already have 4 valve heads.
Larger exhaust may be required, changing the tone of the exhaust. The whole sound of the boat could change. Risking the fish luck.

What noload/load rpm are you getting now? Are your governors giving all they can?
What injectors are in them now? larger injectors, injector timing and maybe advancing the cam can give you a bit more with what you have if you have 4 valve heads.
Gear reduction? If it's close, leave it alone. Putting some real science on the props (or new designed) may help find some speed. Search thru the Post threads. I remember some of the guys upgrading their props with great results.

Just a few knots of speed cost money and reliability. Your hp to weight ratio is pretty poor with those Detroit's. Newer mains would be lighter. With the same or slightly more h p you would go faster with better mpg.

Getting rid of the hibachi grill, charcoal, summer play toys & mother-in-law may help reduce some of the dead weight also. 1/2 load fresh water can help if it's just a day trip.

If it's really 671s you want, pick up a pair of running take out 671TIBs, freshen them up and plug them in. Sell your naturals. Minimum down time, maybe cheaper.

71s Rule.

Keep us up on what you decide.
,rc

+1, including gearbox, best advice/dollar value , plus you can recoup some $$$ by selling your naturals. For piece of mind, make sure you get them dyno tested before install. Stick with the 410hp version for longer life, lower maintenance.........

71's have their place. But the modern diesels such as the CAT's, run so clean and smoothly in comparison and a lot more HP in a smaller package and lighter. If I was going to do a swap out, I'd just upgrade, otherwise leave them well enough alone......A friend of mine had a 41' with 8v92s and it was still slow.....It cruised at 21 knots at 1800 rpms. 1950rpms would've pushed it 23-24 knots..... meanwhile I was on a 45' and it cruised at 25 knots with 6-71's

the 45' Had 6v71 TI's, Johnson and Towers.

If it raises fish, run em till da wheels fall off.

There is no sweeter sound than a pair of 6-71's lighting off in the marina at oh-darky thirty

Yup! I love my Detroits. As to the turbos, when the engines were tubocharged, the cyl kits were changed to lower the compression. So even if you do everything else, expect shortened engine life. The sweet spot for the 71s was the TI version, the TIBs just wound the rubber band too tight.
My 6-71Ns make 310 hp. J&T added N 80 injectors and goosed the max RPM a few hundred to get the extra ponies.

That might be a much better way to go for the OP, would be the N80 injectors and changing the rack and rpm's and leaving them non-turbo'd. I believe it still would be a lot easier on the motor than a TI version. Although at trolling speeds, since it's a SF, it might run a bit dirtier at extended trolling.

Mine is a SF also, after hours of trolling, you might get a puff of smoke as you get on plane, but no worse and probably better than higher output engines. Remember, no matter the size of the injectors, they put out no more fuel at a given RPM. If they did, the RPMs would be higher!

Well, if you only run in neutral I guess you could say that.

By increasing the injector size you can inject more fuel to produce more power at a given rpm. That means for example, that you can turn a larger diameter same pitch prop at the same rpm but increase bollard pull, or you could increase pitch and go faster at the same rpm as with the smaller injectors.

That's why I asked what Injectors & head the OP had.
I remember working on a SF with 6-71Ns. Had N60 injectors & 4 valve heads. Yep, slow boat.
This ship was one of those lack of use dock queens I talk about and the exhaust valves & seats did fall apart. When we put them back together, I installed N90 injectors.
I do not remember the reduction & propeller numbers. I know 2 inches of pitch was added, then a bit more with a cup. That was the max those props could take.
We already picked up 3+ kts and still had room for more pitch.
Would snap out & on plane, race up a swell.
The boat sold before we got another prop set and was unable to complete the workup.
At cruise, Maybe 14kts depending on swells (with N60s), to 16kts solid (with N90s) I was impressed and still under propped.

I'm speaking about the trolling concern. Increasing the size of the injectors should make no differance at any power level below the maximum of the old injectors.They will supply no more fuel than the governor calls for.

That's not true. The larger injectors would be metering fuel for a smaller time window at trolling speeds and the larger injectors do not atomize or meter the fuel as accurately at such low pulse rates.