We had originally planned on getting a 136″ van for compactness. Or maybe a normal-length 159″. In the end, we found a canceled special-order 159″ extended with immediate availability and a good price. We use these seats for lounging and eating.
Ford Transit Seats
We decided on using multiple “single” seats from a Ford Transit. These were readily available as new-take-outs on eBay and seem to have a reasonable price. And they have built-in seat belts which are more simple and trustworthy than trying to rig up our own seat belts. We also liked that they were relatively compact and easy to move from track to track.
The Ford Transit seats are available in a variety of coverings. We chose some basic vinyl grey for ease of care. The grey vinyl matches the grey vinyl insets on the ProMaster seats fairly well, so they do not clash too horribly. Here is a collage I assembled from several e-bay listings:

Seating configurations
We envisioned three seating configurations.
1) Driving: The seats would be forward-facing on very secure tracks so that their built-in seatbelts would be effective:

2) Facing: This would allow a small table between the seats, and also allow entry from either the right or left side sliding doors:

3) Bench: This would give the most “open” room for moving around, and allow great access to the right side sliding door.

And of course, we can take them all out to have some open area for cargo or whatever.
Installing the seat tracks
To make this layout work, the tracks needed to be laid out and interlaced like this:

To mount the tracks securely is a bit of a challenge. First off, the mounting hoops of the tracks wrap around the back of the track (I assume for strength) and that makes the back uneven. So we used strips of 1/8″ steel strip secured with our favorite SEM Seam Sealer to make an even, flat mounting surface on the tracks.

We also used strips of 1/4″ steel strap to even out around and between the ridges on the floor, securing those with the seam sealer. The seam sealer just keeps these shims in place until the holes could be drilled and the bolts tightened — it is not used as a structural component. However, it does do the job of sealing and hopefully will help in preventing any rattles or squeaks.

Where the tracks were interlaced, we made sure the “driving” tracks were uninterrupted for strength and safety. The sideways tracks were cut to allow them to fit interlaced in the same area. We used the 1/8″ steel strip on the bottom of the cut tracks to make sure that their alignment is kept in place during installation. At the lower right of this photo, you can see one of the backing plates used under one of the “driving” tracks.

Under the van
Under the van, we made backing plates . . . essentially huge homemade washers fashioned out of 3″ wide 3/16″ steel strap. We used generous amounts of the SEM Seam Sealer to make sure everything is waterproofed and rattle-proofed.

For the driving tracks, I used beefy 1/2″ grade 5 carriage head bolts in the seat track factory locations, along with an additional 3/8″ grade 5 carriage head bolt at the unsupported end. For the cross-tracks, I generally used 3/8″ carriage bolts — however, some locations ended up inside a boxed part of the frame, for those I used 10mm bolts into rivnuts. For the rivnuts, I used this nice rivnut installation tool.
After all the tracks were laid and secured, we used 3M rubberized undercoating to protect all the bolts and backing plates under the van.
Here is a shot of the backing plates for the seats in the driving position, showing the undercoating:
