I bought a set of second row Honda Odyssey, conversion van take-outs, new modern seats that are very comfortable. I'm a fairly advanced level home fabricator, so I modified the bases to accept the Honda seats, myself.
For the driver's side, I cut the Ford factory base, fabricated new pieces to accommodate the wider Honda seat adjuster track width, and thicker base, welded them together, and bolted the Honda seat in like a factory would, using the Honda tracks.
For the passenger side, I modified one of Ray's swivel bases to accept an Odyssey seat as well. Again, design, cut, drill, weld, grind, paint to match. After I had a design, I think I spent 5hrs per side doing the actual fab and finish work. 10hrs later, I now have seats that are safe, secure, and comfortable. I can spend 10hr in the driver's seat, the passenger side swivels around while camping. A huge improvement over the aftermarket, crush velvet, once plush (now worn out) conversion van seats they replaced.
Very few DIYers should tackle a project like this, the seat mounts are equally as important as the seat belts, in an accident. I used to do auto upholstery as a hobby business, and have seen some dangerously unacceptable botch jobs.
My advice is this; If you can find a bolt in solution for less that $2k, you are money ahead to just do that. If you have your heart set on Sienna seats, which are damed comfortable, there's always a way, but it turns into a project.
1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift