The non-disclosure of the salvage title is a bit of 'dirty pool' but then again she may not have really understood what that means.
But as long as you can plate it when you're ready to ride, the price you paid is perfectly reasonable.
I worry more about salvage on auto titles, particularly modern cars with lots of computers and electrics. On an old analog bike like your CB, it's less of an issue because things like damaged frames are much harder to hide.
It might limit the value on resale, but the '77 CB750 isn't as valuable as the early bikes anyway, and if you end up going the cafe route, or having to rebuild the motor, then the salvage nature of the title won't be that big of a deal because so much of the bike has been replaced.
You should see if you can find out why the bike was salvaged and how long ago it was. The title should have any issue date on it, so you will know that the salvage dates at least back to then.