seat rail

[Bonzo]

I can't remember the exact IVA regs for seat mounts.

I think you are allowed to bolt directly to the floor provided you place some compensation plates on the underside of the floor.
Minimum of 100mm X 100mm is lurking in the back of my mind.

Personally. I would fix some 40mm X 3mm strip steel under the seat runners.

Mr IVA man will most likely be happy to see the compensation plates too.

If you need to gain a little height, then some square tube might be more suitable.