[MrForbes]

I don't see the need for doubling the arm, or using 4x4s for posts to support the pivot....since the arm is only 7' long. If you do reinforce it, I'd suggest putting the reinforcements on the top and bottom instead of the sides. In fact you could make the whole arm from a 1x6, taper the long end so it's about 3" wide at the tip.

The pivot shaft looks a bit long and skinny though.