First of all, the P909 is a good choice for your Serpent 960. The body makes the car stable and easy to drive. If you ask me I would go even further than the 11mm, but it depends also on your driving skills. More forward mounting results in more steering, and personally I like that.
As for your rear arms, I am not sure (since I am a Kyosho Evolva M3 Evo driver), but the max. body height should be 160 mm under the current EFRA rules. Always try to get close to that max. as it will give more down force and the side of the body also helps to keep the car stable in long & fast turns.
I also prefer to mount them when still clear, actually I never use a template, even if I have the same used body. I always start from scratch. This give you the chance to mount it perfectly on your car (as forward or backward as you want it) and perfectly centred. Unscrew the cooling head of your engine, take out the fuel tank, and the roll bar. After making the holes for the body posts you can press the body on the glow plug and you will have your centre point for circling out your cooling head circle.
Take your time to make it fit perfectly, you’ll appreciate it on the track later on.
Serpent 977evo - Ultimate Nitro Pro