There are certainly D rings. Six sounds right. But none aft of the second passenger seat. Hence the need.You should have a bunch of d-rings - six on the floor and two on the b-pillar near the floor.
Do you know the part number for the anchors you mention? Definitely a worthwhile upgrade in my book!There was an option- the cargo net. That would have also fit anchors into the grab handle/vent assemblies. I find ratcheting tie downs often preclude the need for upper anchors. But since you say your transporting art, I assume that was already dismissed out of hand for its brutality.