I'll agree that square tubing is the way to go. I think I made the side rails 4" high, which was enough support to run the top one down at a diagnol to meet the front bar which I put about even with the top of my windsheild. It was also low enough that I could put stuff on top from the side. I see a lot of the bigger cabbed trucks actually run the side rails the entire length of the truck & bring a leg down to the front bumper. I didn't think I'd like that nor did I ever find I needed that kind of strength at the front end. With really long boards, I'd let 4' or even 6' hang out the back rather than put too much weight up front. It didn't come up too often anyway. I'd talk to a real welder, but I would think that 8" high side rails would give you the strength to go over a crewcab with the top bar coming down at a diagnol. I would corner brace the front on the horizontal plane. A lot depends on the size & thickness of the square stock you're using.
My legs were connected to the front & rear of side mounted tool boxes & sat right on the top of the bed sides. I've seen others who don't have tool boxes put the legs into the vertical pockets in the bed, but they need to reinforce & bolt through the pocket. I heard of a guy who didn't & had a heavy load pop the bottom out. Not sure if that was true or not, but if I were doing it that way, I wouldn't like the legs to be a sloppy fit at all.
The best removable bars I've seen were on a friend of mine's truck (co worker, too). He used a larger size square tube with the top cut out mounted to the sides & laid the normal bar into the pocket formed by it. He drilled through & kept a snap pin (one of the straight steel pins with a long, semi circular piece of wire that wraps around & catches the front.) He had 4 cross bars on an 8' bed & did that to the back 3 of them, so that in theory, he could use the entire bed for a tall load. I think we did it once in the few years we worked together for a refridgerator when he moved. It was certainly easy to do, but we did have to put the back bar back in after we went down the road a piece. The back of the rack was flopping around too much. He had a couple of ladders side mounted that we had left on.
Maybe I'm too picky, but after years of working out of a van & then a truck with a crappy set of racks, when I built mine, I wanted exactly what I wanted. I can't weld aluminum, so I figured if I wanted to make alterations, it had better be steel. It was good I did because my original side mounts for ladders & such didn't work out. I cut them off, made new ones & was happy after that. I practically lived in that truck between 10 & 14 hours a day, 6 days a week. On the 7th day, we often used it to pull our old horse trailer. Anything I didn't like quickly became a major annoyance.
Hope this helps, Jim