Perhaps a bit more info would have helped.
A. There would be no additional weight aft as the fuel weighs less then the water it displaces, ie: the tank full would float if detached.
B. The tank would be only a hundred or so more gallons then the lazerette tanks I am eliminating
C. The two 340 gallon main tanks would be eliminated thereby greatly reducing the total weight of the vessel
D. You have to take into consideration the overall minor weight/issue as we are talking about a 60,000 boat
E. Armstrong makes brackets for small 5-7,000+ fishing boats that carry two, three and sometimes 4 350hp outboards
F. Armstrong advertises the added buoyancy and increase in speed and reduced fuel burn when using their brackets
G. Copying someone like Armstrongs attachment design would be using a well known and proven design.
Helps but doesn't change my opinion.
A. If the tank is then displacing water, you just put tremendous pressure on it and on your platform. The risk to platform and boat is significant. Typically you'd need to reinforce the means by which the platform attaches to the transom. But also you might put too much stress on the transom itself.
B. A few hundred pounds becomes many hundreds if not thousands when now moved beyond the existing water surface. Think of a see-saw.
C. Don't know where the 340 gallon tanks are now but this may add to the problem rather than reduce. You're now adding weight beyond the stern and removing weight forward some distance. So could be impacting the distribution of weight even more.
D. Not as minor as you think. Changes the entire weight distribution. I've seen batteries moved make from behind the engine to in front of it make significant change. Shifting 4000 pounds from somewhere I'm guessing forward of the engine to outside the existing boat is not minor even to a 60,000 pound boat.
E. Yes they do. Doesn't mean those boats work well with that load. Doesn't mean they aren't dangerous. Depends on the boat and what it was designed to accommodate. As to small fishing boat, I don't know any small fishing boats designed or approved for 1400 hp.
F. Advertises. Key Word. They show two examples. I have no doubt it would increase speed just like trimming tabs way out. As to the flotation chamber of the Armstrong, I believe perhaps if the exact right combination. Also the right mounting and transom strengthening if necessary. They also looked at boats that did not have good attitudes before the install so started with bad situations. Easier to look good.
G. Armstrong's design isn't well known and well proven for a boat like you're talking about putting it on. Actually the lightness of the boat makes putting the extra pressure on the transom much less of an issue. Now you're putting 60,000 pounds of weight on an extended surface not originally part of the design.
I think making such a modification as this without a naval architect checking all the factors would be a serious risk. Could it work? Possibly, of course. Would it work? I think a significant possibility of problems. Fact is we can't know for sure what the results will be.
Some boats work well with extensions. Others don't. They all need an architect working to determine what the new boat will be like.