Montanas are not even hardy for me in my zone 6/7.

Clematis terniflora (Sweet Autumn Clematis) seems to fit the bill.

Other so called 'fragrant Clematis' do not have a strong fragrance, just a subtle one.

Maybe, you should opt for a climbing rose?

Some of the viticella hybrids have nodding flowers that would make them more visible on an arbor. The only one that I know to be fragrant in my climate (a humid one) is Betty Corning. Take a look at the thread below on the Antique Rose Forum. If you scroll down to about the 5th post, harryshoe z6easternPA has posted a lovely photo of Betty Corning - he says the arbor she is on is 8 feet tall so you can see that she can cover a lot of space even in a colder climate.

Here is a link that might be useful: clematis thread on Antique Rose Forum

Here is a photo of the species viticella growing on an arch. The hybrid Little Bas has a similar color and somewhat more flaring bell shape than this photo of the species.

Pendragon has a reddish purple flower, similar in shape to Betty Corning.

Walenburg has a nodding reddish purple with white flower, but not really bell shaped.

All of these are viticella hybrids that are quite vigorous and bloom prolifically, but Betty Corning is the only fragrant one.

Here is a link that might be useful: species viticella growing on an arch

The arbor is quite large, 10 ft high, 10 x 20 ft on top. I need plants that will go 17-20 ft in height. the viticella are a bit short.

I have figured on 4 plants, I've sussed out the other 3: table grape, non-invasive honeysuckle, &, yes, a climbing rose :-)

It looks like the sweet autumn is not so invasive in our climate, that may be the one.

Hopefully, I will be in the same position someday--getting a vining plant to cover a pergola. Some suggestions on other plants you might consider: for great shade, there's Dutchman's Pipe (aristolochia durior), cardinal vine (campsis repens), and wisteria. All are perennials. Silver Lace Vine is not recommended as it draws too many bees to outdoor living areas. Good Luck!

petunia, I'm trying to *avoid* vines that eat small children & dogs ;-)

How about Huldine? I just planted this one a couple weeks ago so I can't comment yet. I don't think it's fragrant though but definitely hardy enough and vigorous enough! :)