If you guys rent out some VAN/ Jeep from Rawalpindi then this trip would be easier.
I am not sure But there are some APV,Jeep (Pajero) and Hiace available for rent in (Manshera),all you have to do is to take care of driver.
I recommend you should stay for one night in Mansehra and hire your own transport from there.
reach Naran and stay one night and then Naran to Gilgit via Basbusar.
If you haven't visit Naran before then dont miss the drive from Naran to Babusar with breath taking views.