If you've not experienced a Honda CB before, they are great bikes.. The 450 might actually be a better bike than the CB750 for your GF, depending on her size- and it sounds like she is already used to a smaller, easy handling Rebel 250. The 450 would be lighter, easier to manuver at slow speed, and less imposing than a bigger 4 cylinder.
What I hear is, back in the day, the CB450 was the rocketship among the Honda CB twins of the '70s. Even today, considering it's top speed is 95mph, I'd think you'd have plenty of speed for highway driving. Back then, a 450 was considered a pretty good sized bike! It's heritage is the "Black Bomber" from the 60's, which was designed to be a competitor to the '60s Brittish twins.
I personally think these bikes are gems, and would love to find one. Wish my wife would ride!!! If you get the things that need attention fixed properly, and put modern tires on them (I used Pirreli Sport Demons on my CB550K) it will be hard to find a better bike for this use (relative newbie to more advanced rider)