Removed side panel and this is what you get behind. Inertia block will fit in void.

Highlighted three existing holes which will use to mount inertia block. Made some stiffening plates to go either side of vertical panel to support inertia block. The one hole and two bolts to right of timber is the stiff and mounting point already in for the original lap belt.

2 stiffening plates. one 50mm x 160mm x 8mm thick to fit inside void. Other only 2 mm thick to spread the load.
Central hole to mount inertia block is 13mm outside holes are 10mm. Drilled out holes in vertical panel to match plates made.

Plates and inertia block fitted. Used 8mm plate for inside void as this gave a flush surface to mount inertia block. Bolts will be hidden behind panel when refitted.

Had fitted the top connection earlier when trying static belts in so have no good photos of this. Will try and describe what I did.
Made a 6mm thick plate 50mm x 50mm and drilled and tapped this for seat belt bolt. Removed the exterior air vent from van to give me access to behind panel.
Removed carpet trim from inside.
Picked location to drill knowing I could access back to slide in plate. Drilled 13mm hole. Slide in plate and fitted bolt. To make this permanent drilled two holes above and below for pop rivets to make the plate a permenant fixing. You can now remove bolt without losing backing plate. Refitted exterior vent and reglued carpet trim inside and now ready to complete fitting of belt.