Hi, my car has two horizontal 1 " chassis rails, my seats fixing do not line up with these. Would it be OK for IVA to bolt 2 lengths of 4mm steel bar to these at 90 degrees and then bolts my seat to these using 8.8 grade bolts or higher.
Thanks

I don't believe 4mm flat section will be rigid enough at any length to support a seat. The Tiger book describes a similar seat support but they use 3/4" x 2" tall tube. I'd use 1" square tube to match it to the chassis rails and keep things at the same height.

I did similar, ran two lengths of 3mm angle underneath the floor, between the two horizontal rails to form seat supports.
I'm sure that 4mm bar will be fine for IVA tbh, but thinner box or angle is probably better.

Have you got a steel floor panel? My seat runners are fixed with M10 bolts through the panel with large steel (penny) washers on the underside, not questioned or mentioned at SVA.

Hi, I have an Ali floor, so i am going to bolt the seats to 1" box section then bolt that to the chassis cross members and see how it goes. Thanks for the advice

my composite seats on mk2 stoker were bolted through the floor pan plus two stabiliser angles bolted to the side at shoulder height and they were happy with that.

you could have bars across the base for added strength but i image any edges or nuts would have to be radiused.