Is the only reason they said 172 because of the substantial step up?
You're a monster, I had a Never Summer Legacy 174W, usually rode about a 162 @ the time. It took all of 30 seconds to get used to. I weigh in @ about 170-80lbs, just under 6 feet & I rock a pair of size 9 boots.
The jump you'll be doing isn't as bad as a regular width 162 to a 174W, & I didn't have any problems.
You're like 12 feet tall & 400lbs, I don't think your going have any trouble getting used to & throwing around a 178.
If you owned a 169, the 172 will feel pretty much the same.
I was riding 167 wide board, then upgraded to 174 NS legacy-R. No problems at all, even easier to turn and fun to ride.
Now i got Donek 180 with 28cm waist, standing next to me. And i bet i will have no problems with it at all.
It depends on what you gonna get: float in powder or stability on a hardpack. You can actually get both =)
But for float you need super-big and wide board for 400lbs, and for stability it gotta be suuper stiff. Don't avoid big boards: they're fun =)