Make one sheet (4x8) and do a pattern. Finally check outer size
Make 8'6x20 sheet. Make a cut out of 4x8 and do a pattern
looking more for a way to lay the sheets in a pattern with the less amount of waste per sheet
so one can place support under the joints,(angle iron) for support.
You will have to work it out by hand so to speak. What Deepak suggest is what I would do. SolidWorks does not have a feature to automagically do this for you. You can buy nesting programs, but I suspect that is out of your budget for your project.