Iplayfarmer.
I would recommend that you use the 1/4 wall tubing. I would also beef-up the connecting joints with 1/4 or 3/8 in steel/ Otherwise, around each pin area, you would have the 1/4 in wall of the tubing, plus the added stiffness of the 3/8 steel . If it is going to bend or crinkle, it will do it at these places. My front hoe is mostly built, just waiting for some small stuff, and trying to figure out the 180 swing. I have posted some pictures before.
To slow down the hydraulics, just add a ball valve, and adjust it for the speed you desire. That way, you can run the engine for max breakout force, and swing at a slower rate.