175mm2 doesnt sound big enough from what I was told when I did mine. The plate that I put in the pillar was 55 x 70 mm giving 3850 sq mm by 5mm thick. this is the minimum size that I had to put under my seats and also for the bolt oints to the floor.
I got the grade 8 seat belt nuts and welded them to the back of the plate first, then cut the appropriate slots in the side of the pillar to fit.
I then drilled 4 holes in the pillar around where the seat belt bolt would go through, this was so that I could weld the back plate into place.
Placed the plate in place, welded it up to the pillar and then filled in the slot after the engineers was happy that I had done the right thing.
Hope this helps.
Jason.