How would I cut an angle greater than 45 degrees with a circular saw?
I would cut at 30 degrees with the saw table on the end face of the board. This is a bit precarious due to the small work area on which the saw will rest, so be sure to use a stable platform and be safe. 

For this case it's not necessary to cut to a full taper. A typical circular saw cuts to 2-1/4". If a person cut that off one side of the board, it would make a fine joist. I'd probably scribe a line 1/2" from one edge and cut on that. 

Use a straightedge extending along the outer face and beyond to position the joists on the layout line.