1. Make sure you have only an axial load in the angle (no rotation).
2. Anchors are designed for shear only for your application with only an axial load. Make sure you apply the appropriate safety factor.
3. Assuming these are "post installed" anchors and not cast in place, you can use the capacity information from the anchor manufacturer (Hilti, Simpson, Powers, etc.). Would assume this application you would use a wedge anchor, not a chemical anchor. While either could be used, it is easier to install a wedge anchor in a horizontal application.
4. Pay attention to edge distance limitations. These are critical in shear applications. Spacing and edge distance requirements are given in the manufacturer's literature.
The only reason you would need a plate behind the angle is if your edge distances and spacing won't work with just the angle. In that case you would weld the angle to a place sized to accommodate the required spacings.