Garrobito, the numbers you've given above don't jibe. But instead of trying to calculate power, just do like ski mentioned and measure your power at various speeds. From that you can extrapolate how much is needed for V/L 1.34, if you really think it's necessary to reach that speed.
BTW, you are exempt from having to install a T-3 engine as long as you trade-in to the dealer (don't sell the DD engine on your own unless you can prove transfer - check this with the dealer first). A good ole mechanical T-1 will get almost 10% better fuel economy. And absolutely stay away from glow plug engines. That Yanmar is the wrong solution.
If dependability and parts availability are priorities the 6-71 at 170 hp should work just fine, parts and mechanics who can diagnose and repair are pretty easy to find almost anywhere. I would recommend that you use the boat for a years or so before making a decision on re-powering.
..Problem I want to go north with that trawler... I don't know if I can trust an old engine while I'm 70/100 miles off coast with my family... re powering is gonna add a safety key to our trip...
Second point is Detroit is awful expensive on diesel.. new engine is gonna give me at least 10% less consumption (depend conditions off course )
By keeping the same engine you avoid all the changes accommodating and fitting a different one, from mounts to instrumentation and everything in between...my detroit looks much better with an overhaul...
I would be running the 6-71 for at least a while. The payback on all the "Fuel Savings" would be years away and probably not under your ownership if you consider the repower cost. 6-71's are loud, dirty and not so fuel miserly. If its run at 1400 RPMS it is not so noisey, not so loud and very fuel efffcient. Detroit engine longevity is legendary.