(Oh no! We have Menards here!
)
That's a beautiful arbor, Sandy! I'd second the strong trunk advice. Also, you might consider doing a wrapped trunk first
http://garden.org/blogs/entry/... and then once it pretty much supports itself, let trailers grow up onto the arbor. That way you could cut it back every year if/when it gets out of hand, or if you need to do maintenance on the structure.
Otherwise, I'd set the arbor in a concrete slab. It's entirely possible that it would eventually pull it off-center if not.
Wisteria can be a long-term project, so there's no need to feel that several years worth of trunk-building will be wasted time; I think most of them take that long to start producing blooms anyway.
The flower buds on ours get killed by late freezes almost every year...what a waste! But yours is a different variety so hopefully it'll set buds a bit later for you.
