it's reasonable, don't look like anything will be too thick.... and yeah, use the door as a guide for the long board..

go take a large framing square, and put it flat to the rocker and see how the qrter edge looks... does it progressively get wider to the top from the bottom, or is it mostly at the top... even a yardstick along that qrter edge to see if it's straight..
the qrter doesn't need to come forward at the top, the post needs to be bent forward.... could the very back body mount take a couple washers without f*ckin up the tailgate?
take a closer up pic of that door to qrter gap if ya can..



go take a large framing square, and put it flat to the rocker and see how the qrter edge looks... does it progressively get wider to the top from the bottom, or is it mostly at the top... even a yardstick along that qrter edge to see if it's straight..
could the very back body mount take a couple washers without f*ckin up the tailgate?