On Spitfires and GT6s with the frame and chassis being separate you mount this main hoops a bit differently, you'll need flat pads on the bottom the main hoop that serve as attachment plate that sit in the floorboard, with a suggested (4) 3/8" grade 8 bolts securing them to,and thru the floorboards with thick back up plate the same general size as the maouting palte on the top side used below the tub where the bolts from the mounting will be nutted, this is required by the SCCA so the cage will not pull thru the floorboard, you can also weld the mounting plates along the edges to the tub on the top side to give addtional security and still have a tub that is removable from the chassis if needed for futrue work or repairs. Hope this helps.