I wouldn't use angle iron for that application. It is not tortionally rigid enough. Basically, angle iron likes to twist if loaded like that will be loaded. I would suggest using round or square tube instead. 1.5" x 0.188 square would be my preference. Or maybe 0.120 wall as long as you spread the mount area out a bit (tie into the horizontal walls), just welding on tabs might eventually crack 0.120 wall due to fatigue.
edit: I should also say I agree that in-boarding is not the best choice. You loose all roll control. It is popular only because it is easy to get major travel that way. A better solution is kicked forward along the frame rails. You need a very stiff shock due to the angle, just as you do for in-boarding, but it will help control roll rate. Even better is to go up through the floor vertically, but most people don't want the hole into the bed or the supporting structure.