I was going to say 'have a look at my thread for photos of my alfa seats into viva' but I have only just realised a lot of photos from my Viva thread are missing thanks to imageshack being greedy. I will have to re-do them on another server.
anyway- I re-did one photo so you can sort of see the shaped plates and raised mounts to suit the Momo (Recaro made) Alfa seats. I had to use 3mm plate and 5mm plates underneath to keep the engineer tester guy happy when it came to getting certification. But its well worth making them strong as they dont move at all and feel well solid in place..
Here's my mate Dave working on them...
On to the top of those plates that were bolted to the floor I welded some 40mm box section capped with a bit of flat. Under the caps I had welded nuts. I made a wooden jig that copied the seat mounting holes. This I sat on top of my raised box sections and I could cut them to the right height before welding. I used a spirit level on the wooden jig which made an easy job of getting all four corners the same height. Sadly no photos of that.