Carl,

It is difficult to give you accurate dimensions without knowing the configuration of your seat runners and the distances between the runners. I had the same problem with my TF.

What I did was put the body on the chassis and put a strong light underneath to show where the chassis rail ran and marked them with a marker. I was then able to position the seats in the tub to the optimum position and work out what steel I would need to bolt the seats to. 3mm steel plate would be sufficient if you are welding both ends of any steel you are putting in to fix the seats to. For a single ended welded fixing very much depends on the length of the bracket you are using. My recommendation would be to weld both ends of any bracket you are using to 2 separate places on the chassis..

What are you going to do about the inner seat belt fixings? I fabricated and fitted a cross member to bolt the seat belt too. As I understand the IVA, unless you bolt into steel or have a reinforcing plate of at least 4"x4" with the hole in the middle it won't pass.

If you look at the links on the web site, Chris Humphreys has done some really good work to document the work he has done to build his car which includes some diagrams/pictures of the chassis mods.

Alternatively, I am quite happy to talk you through what I have done and send you some pictures. Just let me know on my email address.

Regards

Paul