Hi what you need is too mount them basically as far back and as high up as you can on the main floor pan so any force that gets trasmitted to them is a more sideways force or as low an angle as you can, Ideally mount them in the floor through as solid and thickest part of the floor you can. In the Cars I worked on ( Rangerovers Bmw's The new Mini, Rovers, Some Hondas and some Nissans, I was a Engineer in charge of Reworking any assemblies that didnt meet standards back too Engineering specs) we had a plate nearly an 1/8 thick and sometimes 3" by 2" Square that was spot welded inside another Assembly then this was mounted again with multiple spot welds too the floor, So basically In the event of some Force it spreads that force over a greater area and wont pull through ( the last thing you want too happen).More modern Seat Belts are mounted that will Peel slowly allowing force too be built gradual. but its a bit precise Engineering For that, My personal approach would be too weld at least a 1/8 inch 3" by 3" too Either the head of the bolt and mount it With bolt going inside the cab then Either a larger plate or washers on the inside under the seat belt, Or weld a 3" By 3" plate inside too the floor (Less desirable) . All you are trying too do is spread the load over a greater distance and prevent them from pulling through the floor. On A personal Note I hope what ever way you mount them That you never ever need them
Ryan
