I have just completed my 550 to 700 stretch for AP work I did this after trying 600 stretch with belt and Torque tube and 500 belt trying to get the Tail perfect for filming Video.
After the first flight I I'm very happy tail is very solid with the least amount of bumps I have scene I put that down to it now being so far out of the main rotor wash.
So parts used standard Trex 550 700 tail boom cut a few centimetres
to allow 700 torque tube to fit with 550 mount.
and I run the DS650 tail servo mounted to the boom connected with a carbon tail rod.
Blades are 550mm Align I have tried 600 but MX motor or ESC have trouble so until i upgrade to Scorpion/ ICE 100 conbo
I will stick with 550mm I have no need for more lift at the moment
I run my own Camera mount and a Canon 7D camera
view some of my Helicam videos here http://www.44magnumproductions.com
Happy to answer any questions.
will run if chased