Are you familiar with the 3DR drawings of the 'B' chassis? (It looks like they pulled it from the 3DR store - not sure about redistribution issues of IProp though it is out of production.)
I see you found my sneek peek of the 2nd build - clever fellow. I have machined some material off the ends of the arms since that photo. The point was to make it more visually appealing and to provide easier access to the m'otor mounting hardware. The 'C' version of the 3DR chassis puts large access holes on the underside of the arm. Fishing screws into those will be difficult so why not remove the entire area anyway? It serves no structural purpose.
I will sketch you us something on the orginal dimensions and post it here. The 'C' chassis has slightly shorter arms than the 'B' chassis.
The 'B' chassis had the following dimensions:
Arm tip to arm tip: 25.45 inches (central frame plates are included)
Individual arm length: 11.02 inches (another odd value - round down to 11.0)
That should get you moving towards something that will work with Arducopter default values.
I would not go so far as to say weight is not a 'big factor', I wouldn't dwell on it for your *first* quad.
There are so many things to learn and do that you should strive for the least amount of weight possible. Just don't fret over tens grams here or there. Hundreds of grams...you should think about. Kilograms...definitely rethink.
I don't have a GoPro but use a Philips 'flip' type camera. With it on the quad flight times are in excess of 8 minutes... close to 9 minutes.
Balance? - gross off center weight will require some serious trim (pitch I suspect). The autocal mode of the AC code may correct for this but you still have to provide the manual input during the event to keep it level. Of course the harder the bird has to compensate, the more energy it will take from flight time. (See the comments on weight).
Here are some weight saving ideas I am trying on the 2nd build:
I have been sidetracked developing several things of late including a very different set of landing gear for the 2nd build. A major side track has been cutting parts for my new CNC. The first one was employed for several quad parts but the new one should be able to cut the aluminum arms too.
Original 3DR arms are 11 inches. (Did you see my earlier reply?)
The arm length on my 2nd is 9.75 inches (but the central body is larger and keeps the tip-to-tip length the same as original).
I know i can build the frame as many times as i like, however i would like to get good gear, and not have to purchase the gear over and over again
Can you please check this out, are these motoes and ESCs good?