Botchan, I wasn't able to get decent photos of the base in my van, but below is the assembly manual from Base Fabrication, LLC, the company that manufactured the base adapter. I bought the base adapter from Scheel-Mann and it was shipped directly from BF. I hope that helps.
Both seats, passenger and driver swivel. Personally, I think it's easier to swivel these than the stock seats they replaced. As with the stock seats, you have to move the setback forward (de-recline?) in order to swivel around. I was a little concerned about this aspect, because the Scheel-Mann seat backs recline via a knob, rather than the lever on the stock seats. But, thankfully, the screw has a course thread, so they move very quickly with the added bonus of infinite adjustment.
1. Seating height: I first noticed the seats sit higher from the floor than the stock seats. This was a welcome surprise because, as I mentioned in my OP, my problem was always my legs, not my back. A higher seat height will allow more distance for my legs to stretch. Before, I was sitting almost Indian Style, now I can point my legs straight at the pedals with a small bend at the knee - JOY!
2. Femer position: Going along with 1, above, instead of my femer (leg bone between the hip and the knee) heading down hill, my femer is at a slight incline. This helps with leg room, because it elevates the knee and puts it farther from the pedals. I'm sure there are other ergonomic benefits.
3. Armrest (option): The armrest angle when in the down position is also adjusted by a screw, so is infinitely adjustable. It just so happens, my right arm feels like its at the exact same elevation and orientation as my left arm, which rests on the door arm rest. I think that will help me from feeling twisted on long drives.
4. Seat Bottom Adjustment: Another thing to help my poor legs is the fact that the seat bottom extends and contracts forward or backward with the use of a screw. This will help provide more support for longer femers, like mine.
5. Under-seat storage: As an unexpected bonus from the mounting brackets (I think), there's more storage under the seats. Anything I store on the floor under the seats will be easier to access, but there's also space between the swivel and the seat bottom, which would be a great place for maps, books, binoculars, etc.
That's it. Honestly, I had heard many positive reviews of these seats, almost to the point of worshipful. I was desperate and these seats appeared to be the closest I could find to a solution. I'm becoming a believer. I admit I haven't driven any long distances, but am very encouraged. I'll be driving it from Wisconsin to SW Missouri in early May when we move down there - about a 10 hour drive. After that trip, I'll try to remember to do a write up, but I expect it will be hectic.