PizzaMinion
At 6'4", 325, I have learned that not all frames, wheels, etc. can handle the forces exerted by an XXL rider.
My LBS is building up a set of Velocity Chukkers with White Industry hubs, but I'm curious to know what other options (hubs, rim's, wheels) are out there.
I've looked at Spinergy (no weight limit claimed), Rolf Prima (over 500 lb weight limit claimed), Astral Leviathan Tandem wheels (over 500 lb weight limit claimed).
Industry 9, Mavic, Zipp, and DT Swiss are all rated between 250 - 308 lbs.
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BubbaS
Bontrager also claim no weight limits on some of their carbon hoops and offer a nice looking warranty.

BS
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Perks Tooth
You are doing the right thing by having a wheel built by hand. For gravel it should be at least 32 spoke with a hoop that does not prohibit your weight.  Ive struggled with this problem in the past and I weigh 100lbs less so I sympathize.
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avatar78
Yeah I don't think I would buy any prebuilt wheelset. Definitely handbuilt with quality rims, hubs, and J-bend spokes, 32 or 36 count, 3 cross all around. You're doing the right thing getting a wheelset built up. Look at the components Zinn Bikes offers for their wheels for good stuff to use, tandem wheelsets are another good area to check. 

Zinn:
https://www.bigandtallbike.com/Wheels_c_79.html

Edit to add: I haven't visited it a lot recently, but the Wheels and Tires section of the roadbikereview.com forum typically has wheelbuilders actively posting, and can give recommended wheelbuilders if your local guys can't do it for whatever reason.
https://forums.roadbikereview.com/wheels-tires/
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DIRTBLACK
You mit try a tandem wheel set ,they are build to take a lot of weigh and force.
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PizzaMinion
Thank you everyone for the guidance - its greatly appreciated.
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TrailMasonCliff
I feel your pain I built up a wheel set using Spank Ozzy295 rims and DT350 hubs with Sapim laser spokes. They have been true and perfect for the last 3000 miles of Kansas Gravel under my 6'5" 330lb Clydesdale ass. Broke one nipple (aluminum) when I got a stick through the spokes. I was playing MTBr and enjoying a very overgrown abandoned road section but finished the next 30 miles of the ride with only a small wobble, replaced the nipple trued it up and still running strong. 
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