I'm not aware of any Rhododendron cultivar being known for a narrow, upright shrub shape. There are some, like R. arboreum, that have an upright shape overall, but 1) they aren't terrible fast growing 2) they won't stop at 8' and 3) they probably won't look good hacked to 3' wide.

I'd recommend you post to the CA forum or shrubs forum for a more general recommendation. In SE AU or NZ I often saw nice hedges of various native Pittosporums. Someone like Sara Malone would have scads of other recommendations.

If you must have a genus Rhododendron plant the only ones I've seen having close to that shape are certain lepidotes like 'Trewithen Orange'. Sonoma Hort. used to have it and various other rare-in-the-US lepidotes...I don't know where else you'd buy them locally. A couple PNW nurseries carry it. Lepidotes are the 'scaly' leaved rhododendrons with smaller leaves. Sound unappetizing but the scales are almost microscopic! And often just give the leaves a matte or slighly iridescent look. My 'Cerulean Mist' isn't very big yet, but looks like it will have a fairly upright habit too.

Living in the land of giant rhododendrons, I would have to echo David's contention that a tall, narrow species would be highly uncommon.......that is just not by nature their normal growth habit and I can't think of any that would readily fit that description. These tend to be mostly rounded or spreading shrubs, typically as wide - if not wider - than they are tall. And pruning to size would not enhance the look :-)

I'd look at some alternative shrub suggestions to fit that very specific sizing.........

All are easily pruned to be narrow, but that's going against it's nature.

Westes would a Camillia grow in the spot you’re looking at? They grow well in certain parts of the Bay Area, have similar requirements as rhodies/azaleas, are readily available at local nurseries this time of year and can fit the dimensions you need.

@Cori, I will consider that. I like the rustic nature of the rhododendron and I want the one plant that I am searching for in this thread to match others that will be planted nearby.