Up to SM/ZF will still be reatively easy with a L64 hunter, but you may as well level him to 70 as you build up a bit of rested on the alt, no?

In terms of instance boosts too, although it may seem counter intuitive, it's a good idea to invite some other low level alts for your boosts instead of just taking 1 char along, as it lowers the average Level of the group and your alt will get more XP.

Good luck with it.