Sounds like the OP wants to cut furings, don't think a mitre guide will do that
You can make a taper cutting attachment that is often used for table legs but at ocer 6ft long it will be too dangerous. Use a straight edge to mark corner to corner then feed it buy eye.
Or tack a 7" wide strip of ply to your 6x2 with one edge aligned corner to corner, set your saw rip fence to 7" and feed the ply/timber through the saw with the ply against the fence, this method will give a straighter cut.
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