The leaf front end handles like a boat on road and the stock seats, which were probably crap when new, were completely worn out and useless. There was no lateral support to speak of, other than holding yourself in the seat using the steering wheel, while cornering at speed.
After a bit of research I found out that surf seats would bolt straight in and the next closest seats were Honda Prelude or Integra.
I didn't really think the surf seats looked that much better than the Hilux ones and I knew Honda seats were quite comfortable so I started looking for them.
The seats I installed are not engineered. I would expect that they could be as I have used the stock floor mount points and bolts. I think the only part that would need approval would be the parts I modified. That is: the rear mounts previously spot welded to the rails are now bolted with high tensile bolts to the rails - I also used washers and locktite.
A Gumtree search later and I had the seats below for $200.
You can see the difference between the two seat rails - Hilux left Honda right
The Honda seats bolt straight up to the stock Hilux front mount points
The Honda rear mounts are left in mid air when the front is bolted up
My solution was to transplant the Hilux rear mounts from the Hilux rails to the Honda rails
Remove the Honda rear mounts. I ground off the head of the spot weld /rivet with a grinder
To remove the Hilux rear mounts I alos ground the heads off the welds. The only difference was that because I wanted to reuse these mounts I needed to also pop the rest of the spot weld / rivet. I used a vice and a socket to pop it out.
You can see how the Hilux mount will line up with the Honda Rail. Not all of them lined up dead straight like this but they were all very close.
Rear bolted and mounted
The end result is a set of comfortable seats with great lateral support. These seats also had drivers side hight adjustment.
One slight problem I encountered with this method was that the bolts used to mount the rear inside mount on the drivers side seat fouled the seat from sliding back as far as it normally would. The seat roller stops at the bolt.
If you need the seat to go back that far (like me) then you can replace the Hilux mount with 45mm square section steel. It's only the driver side inside mount that I had this issue with.
