The plate which anchors the tailgate to your pivot point, if the plate extended beyond the pivot point you could then install the rod from there and anchor the other end to the frame rail. You may have to experiment with location of anchor point on the frame. Vertical may work but having it more forward will have quicker reaction time.
Clear as mud? Go on Y toob Take a look at some Cat 740 rock trucks with a tailgate and you'll see what I mean.
Nice work so far by the way, looks good.
Is the tarp powered? Susan wants a powered tarp for her trailer someday and I'm looking for options and ideas.
Mario