Fitting the Inner Sills
The inner sills are the long thin bits of fibreglass you get with your body kit. They fit inside the car to seal the sides to the chassis. They are very easy to fit.
Its very likely they will need some trimming, don't trim the curved section that fits around the door lip until they are secured in place or you might over do it.
They are generally fixed in with a combination of a few large rivets and fibre-glass. One extra step you can make is to fit them in 2 halves, this allows the front half to be fitted with self tappers to allow it to be removed should you ever need to access the windscreen pillar fixings.
The sill should be positioned such that it overlaps fully the vertical return on the main body around the door, this is because you will later need to fix the door seal rubber to this and it fits over BOTH parts.
This picture shows the rear section that is secured in permanently, this was riveted in place and then secured with West System Epoxy 105 and woven rovings fibreglass roll. West System epoxy is quite expensive but far superior to anything you can buy in Halfords. (2Kg of epoxy is probably more than enough to do an entire sumo and available from most boat chandlers along with fibreglass mating and woven rolls)
East Coast Fibreglass Supplies
This picture shows the front area, there's not a lot for the very front of the sill to attach to so a a strip of wood was secured to the chassis to allow the front of the sill to be screwed into it.
Once the sill is secured in place you should then file it down so it is flush with the vertical return on the door lip, this will give you a nice double thickness lip to attached the rubber seal to. Note: you may need to file down both parts if the lip has been left a little big from the factory. You are aiming for the lip to be just large enough to secure the rubber and leave it nearly flush to the bodywork as shown below. This rubber will then fit snugly up against the door card when the door is closed.
