I'm sure your right as the 764 would require a runway extension before it could operate EDI-EWR with a full payload.
While you are correct, the amount of cargo carried would be nowhere near the 20+ tonnes that the Boeing figures take into account so I doubt runway length would be an issue to EWR.
CO operated B764s from GLA for quite a while, GLA's runway being only marginally longer than EDI's, of course.
The stand/terminal facilities that you mention are the bigger issue.
In theory a B772 could go on stand 6A like the B764, but logistically it isn't a great solution and CO are much better off operating 2 B752s until the facilities are in place, whenever that may be!!