I'm going to say this and don't mean it disrespectfully, as I love trucks too, but buying a dually or even single rear wheel 3500 diesel to tow a 20' boat and haul junk (which I read as trash, some lumber, etc) just feeds your ego, isn't at all needed, and as far as using the right tool for the job is overkill to the point of idiocy, especially a dually.
You've mostly answered your own questions, although vaguely. The rest of us don't know what "junk" you'll be hauling, but a trailer has been mentioned. Trailers can be cheap, and they can save your truck from a beating. We also don't know what type of boat you've got, but most that size I think are pretty light.
I've going to take a guess at this, but since you bought the Taurus SHO, I'd bet you like something sporty/quick. That big diesel truck will be way slower than the SHO, ride rough, and depending on their age and stature, the wife and kids will have a hard time climbing up and in. Sure you can mod the diesel to be fairly quick, but $budget$. Also, the old F150 with a 5.0 will not be sporty either. It'll get the boat job done, although not in the style you'd like.
My recommendation, and I've done similar in the past:
Keep the SHO, buy the F150, put liability only on it, and do your chores with it, enjoy the SHO with its pep and decent MPG, as well as easy daily parking.
Or, if you just don't feel at home in the SHO, sell it, eat the depreciation, and for $20k or so, go get a 1500 truck. You'll get a better optioned, lower mileage, newer truck, and a high 300hp 5500-6000 lb truck will be quicker and sportier than the 8000 lb mid-300hp diesel buy a long shot. I'd venture to guess that empty, the 1500 will get better MPG than the Cummins in the years you're looking at. The 1500 will easily tow that size boat, unless there's something taking it over 5000 lbs.
Carefully evaluate your needs vs wants. Take the wife with you to test drive a 3500. Hell, ask her to drive it a bit. She might make the choice easy. I paid a whole lot while younger to drive a truck that fed my ego but was overkill for my wants and then waffling back to an affordable car and older truck. Whatever you do, make sure you only do it once! I finally got into a half ton that fit my needs for many years until my finances improved and I decided to get a slide-in camper, now I'm into a 2500 diesel and love it, but it's overkill when the camper isn't on, rides comparatively rough, tires cost more, and on and on.
Good luck, hope you find something that both works and makes you happy.
