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I have a 6.7L 400HP/400ft/lb torque diesel F350 and, I agree it might be overkill for your needs, but if you don't want to know your boat is back there and feel the drag, it would be the best option. I have the middle drive axle ratio in this truck, there are 3 different ratios available in the F350. It is not the lowest ratio available, (which allows more power but less top end speed, and thus reduces fuel economy), and it is not the highest ratio available either. This drive axle ratio is combined with the lowest ratio 6 speed automatic transmission included with a HD towing package.

I also have a F150 with the 302ci (5L fuel injected 365HP) engine that I think would probably be satisfactory in your application. I have used it several times to pull HD 30' gooseneck hitch flatbeds around for relocation around both the ranch and laydown yards, but I have never used that truck for these purposes at highway speeds. The above described F150 does fantastic at highway speeds with our Outage 18.
You may notice that there is only a 40 HP engine difference between the 2 pickups I am discussing here, but the GVWR of these 2 trucks is over 100% in difference.
You should not get overly concerned with GVWR on your selection once you determine that this rating is high enough to tow the weight of your boat down the highway. It makes no difference whether it is a 1/2, 3/4, or 1 ton chassis in any way other than comfortable ride. Generally speaking across ALL brands of pickups the ride will be harder as the GVWR increases in rating. Since you have already determined that your boat weighs between 6500-7000 lbs, you need to find a pickup that has a max towing capacity range to match this load. The F-150 has a max towing capacity in 2015 of 12200 lbs. Since you are looking to buy used, you would have to find out what the max towing capacity of the particular truck you are looking at actually has, as this MTC has varied through the years, and also varies between makes as well as models within the same make.

Now, lets talk about what is REALLY important to your stated needs.

Both of the pickups I talk about here have 4 wheel drive and towing packages. There are several rear end and transmission ratios available for both the F150 and F350 pickups. You want to have pulling power at highway speeds. It does not matter what brand of truck you pick, the engine/transmission/rear end ratio all WORK TOGETHER to provide you with this requirement. 2/3 of what you desire to achieve with your pickup pulling your boat has to do with components that are not being discussed in this thread yet.
The GVWR and engine in your truck does not make your truck do what you want it do ALONE. You must consider drive axle ratio and transmission ratio in COMBINATION with the engine HP in order to achieve your towing goal of less engine strain at highway speeds.

You don't NEED a 1 ton truck, or a diesel engine, to pull your Outrage 25, unless the brand you choose requires that high of GVWR and engine type to provide you with the towing capacity you require for that boat. I don't know about all the off-brands mentioned in this thread, but I know that both Ford and Chevy certainly do currently make trucks with almost 2 times the towing capacity you need, AT SPEED, in their 1/2 ton models that DO NOT HAVE DIESEL POWER. Ford even makes a V6 Turbo engine they call ecoboost that provides almost as much horsepower as that beast in my F350!
My Dad has and uses one of these F150 Ecoboost right now, and he is selling his F-250 diesel because he doesn't use it anymore since that F-150 Ecoboost came on the scene.
It is my understanding that one member here, Finnegan, uses an old Cadillac to pull his Outrage 25, (and has posted great performance reports all over the internet), using this tow vehicle on fairly LONG trips compared to your needs.

I suggest that you hook up your boat to whatever pickup/tow vehicle you consider and test drive with it attached before you buy. You can't go wrong with that method, and it eliminates the BS you will get if you try to make your decision based on other peoples preferences which seem to almost exclusively match whatever they have invested in :) Good Luck in your quest :)