1. Your in the right place for some help.
2. Having your shocks ahead of time may be a good idea if its a first build. I had mine before we started. This lets you test fit in the process of building. You can avoid tearing stuff apart later.
3. You are going to be better off in the long run on time, money, labor, fabrication to get the good shocks to start. Mounting styles differ from general auto shocks to the fox, king, etc ball mount shocks.
4. What are you planning for a drive train.
5. Wheel travel will be grater than the shock travel if the shock is mounted away from the wheel. If the shock is mounted to the arm 8" from the frame. and the A arm is 16" long the wheel can travel roughly 16" with said 8" stroke shock. But you are placing a 2 to 1 leverage factor on the shock, it will take more pressure and or heaver springs to control the load.
6. 90 to the arms not necessary in my opinion. You use your pressure better closer to 90.