It is pruning group II the large flowered hybrids that are susceptible to wilt and less vigorous than group III especially the viticellas.

Thank you! I did a little searching, and love the clematis montana rubens. I think its the winner!

The montanas are included in group I - minimal to no pruning. Virtually all the vines included in this category are species and most species will be more vigorous (if defined as large, usually fast growing and hardy/resistant to problems) than the hybrids. But, as BorS notes, the viticellas and the close viticella hybrids - all in group III - are also very vigorous as well.

Are montanas hardy in zone 6?

Yes, for me they are, so far...Rubens & Mayleen overwintered this past year.

Montana reubens grows well here in zone 5 and after years and years, finally bloomed, but not the blooms that I have seen in pictures.
Prince Charles blooms heavily and Pearl d'Azur is a REAL winner here. Both bloom so heavily you can barely see the green of the leaves.

Pruning group 3's are the way to go..I am always amazed at the vigor of Clematis "Huldine","Polish Spirit" ,"Etoile Violette" and my favourite Pruning group 2's are "Franziska Maria","Empress" & "Josephine"..they have all been real winners in my gardens...Jeanne

Betty Corning is a monster. My Huldine and Polish Spirit are young and I can't tell yet.