I'd say start with UK. at 73 you can enter both Azjol'nerub instances, Complete all quests there, should make the traveltime worthwhile, if you have HGWT or can summon. Then you can go to Hall of stone, farm to level 75 and farm HoL to 80?

Personally I don't like the HoL / HoS instances and would prefer Drak'theron keep / Gundrak after the Azjol instances.

In general I really doubt how effective Instance boosting is at that level. Personally, I've done the quest area's quite some times, so I can pretty much race through them. I know which quests to skip etc.

I have no facts/numbers to back up anything though.