I did a double row seat out of a Transit with the integrated seatbelts; works well, but of course not swivel. It does come out easily for extended stays (and doubles as outside bench), but I often just use it with the front passenger captain rotated and the SMB table for workstation, or just to lounge with feet on the rotated front seat. The seating position is slightly above the front seats and gives passengers back there a feeling of being on a little "perch" with great view out front. The integrated seatbelts are "locking" (I believe this is called ALR) for legal child seat installation.
Install was trivial with basically high-strength bolts going through the frame with backing plates under, however for liability reasons I had a "NHTSA certified" installer do it and saved the invoice as well as their advertised credentials. Personally, I would be leery of any DYI seat (and especially external seatbelt anchor) installations, it seems to be a gray area at best.
These also look quite good but are pricey, probably only makes sense if transit rails are already in place and its plug and play:
https://www.jmgsystemsllc.com/ford-t...for-oem-tracks