The issue I have is that it is growing on a temporary metal support that looks like this might be the last year it will do the trick, it's rusting. I am hoping to construct a new arbor of wood in the fall. Since this year's growth will be next year's fruit, I'm wondering if there is any way I can prepare before I have a lot of growth that has to be cut back to accommodate a new arbor?

Not the best angle to see the structure of the vine, but if you can't see it, I can try to get a couple more photos tomorrow.

The main thing to know is you can't permanently ruin a grapevine. No matter what you do you can recover and have fruit in a yr or two. You can change the shape or whatever and they just grow back. So I won't sweat it now.

And once the arbor is covered with branches, what do I cut off every year? I don't have to cut everything back to the main stem every season, right?

Once the vine is on top the system you use is dependent on how much area needs coverage vs how much the canes grow each year. The simplest system is to prune like you would a normal vine. Head off your trunk in the middle of the arbor. Hopefully you then get at least 4 canes off the head. Position as needed for coverage. Those canes bear the next year. The following winter prune back as much as possible while still retaining 4 new canes for fruit the next year.

The difference between normal and on top of arbor is how the canes are positioned. Normal they run down wires in two directions. On the arbor the canes are positioned so new growth covers area needed. They may pinwheel out from the head. You might have two heads depending on shape of arbor and area needing coverage.

Once you get the feel for it pruning grapes is a lot easier than describing it.

Thanks! :-)