I'm currently in the process of fabricating some adapter plates so I can fit my GTI swift (the ones with harness holes) seats into my '85 barrow track.
On mine the rail weren't interchangeable.
The rails on the swift seat are approx 10-20mm wider and much longer. I'm lucky in a way that I have the same bracketing system on the passenger side as I do on the rivers side (courtesy of the previous owner).
I will post pics when I get the photos back and the seats in. (about a week or two)
I put in Honda CRX seats and they are awesome. You can fit a bunch of different seats in your zuke you just have to fab yourself up your own install kit. Took me about an hour.
I sure don't have a shortage of co-pilots with these seats on the trail.