At first glance, here are some 767 routes to Europe:
EWR-ROM, EWR-NAP, EWR-VEN, EWR-BCN, EWR-MAD, IAD-BCN, IAD-DUB, IAD-MAD.
I used FLIGHTRADAR24.COM where you select DATA, then AIRLINES, then UNITED then FLEET (on the lower bar).
This shows the entire UA fleet.
I then clicked on either 767, or 764 and it shows all the Tail#s for each fleet.
By clicking on any Tail# you will see what routes that plane is currently servicing.
May not be the easiest way, but it works and keeps you occupied!
Also, once in the Tail# section, if you click n the FLIGHT # it will show you all the Tail#s used for that route.
Another exciting rat hole to borough down, but hey that's what the internet is for Right?
Hope that helps and have fun researching.