Hey Greg...It appears that you have already gotten a lot of good advice, and have come to your own conclusions as to which way to go...but I wanted to add a picture of my FRONT bench back frame for you to reference. I'm not even sure if the middle seat back frame is the same as the front, but I don't have a picture of my middle seat stripped down to show you...however, when I attempted to clip in my grandkids booster seats to the middle and rear benches, I found some lateral round bar portions that almost fit the clips. So depending on the type and size of the clips that come with your rear-facing, front-facing or booster only seats, they might already be there for you...or maybe a slight modification is all you need? I think you can see what I'm talking about in this picture? Good luck.
Woody
