Also, if you're really sore about not quite making 70 before you're done with the Outlands quests that can be finished with a multibox team, you could rest up and run lower level instances while you're 66-68. The mobs are still green to you then, but you'll want the rested XP I mention. I say do it this way since your team might not be instance-worthy yet and the Blood Furnace (just as an example) is a cakewalk if you're a few levels higher. Nothing screwy in there at all. After that, pick up with the questing. There's enough to go around, but it's still pretty rough. I think you'll be able to do it, though.

As an aside, I can appreciate the toughness of this situation. My mixed group is just hitting Northrend (despite being 72 before arriving since I instance-leveled in TBC content and then did all the kill and group quests from Terrokar on up). I didn't think there was enough quest XP to quite get me to 72, but it worked out just fine. Missing a large part of the XP from kills is a major blow to the group. I guess that's what the instances are for, but dang. I wish Blizzard would stop punishing grouping for what is generally considered solo content. The collection quests aren't even the root of the problem here, but they certainly contribute to it.

Grouping makes the questing go by so much faster for the most part and I don't get why Blizzard wants to punish people for cooperating - especially in such an environment where there isn't going to be loot drama or anything.