Hi,
As Tony says, you will need to reinforce the floor with something substantial to convince the IVA people - unless that is you can find where Beauford got theirs tested! In 2010 I went to Southampton and would advise you not to go there as at that time they were super cautious having passed a kit car pass that was then involved in an accident with serious injury due to inadequate strength on some component – at least you end up with a VERY safe car, if somewhat heavier than most!
I had Jag seats, the front each fitted with an integral central socket for the seat belts. To pass I had to put 50 mm angle iron under the wooden floor front and rear to support the seats, held to the car chassis with M12 bolts and with M12 studding tapped into the angle iron and lock nutted to secure the seats before it would pass. The seat supports were significantly stronger than the surrounding frame and chassis. For the rear seat belt anchors I had to weld in a 50mm square tube triangulated frame each side with a 50 mm square brace across the rear in the boot area (what a joy to weld onto the chassis on a finished painted car!) before that would pass – safety factor must have been in the 10’s of 1000’s!
Probably best to actually discuss questionable issues with the prospective IVA Test Centre as they are helpful and will consider sketches of what you propose to do and inform you if this is adequate. N.B. They will not advise you on what you should be doing, only refer you to the IVA manual.
Hamish