Yeah I did a conversion for the Brapwagon, I went for cheapie Saas seats, not fixed-back, but my main thing was having some kind of side-biolstering, plus I'm very tall so I set the driver's seat about 50mm further back on the frame to get more leg space. I retained the original Volvo seat rails, and created a bracket using 5mm flat bar mild steel and box-section steel. You can see photos here: https://ozvolvo.org/discussion/comment/121326/#Comment_121326