Inner sill has been replaced before but not good enough. 

Here we have the inner sill on the right hand side, this pic shows the seat belt mounting behind the passangers side.Here we have the inner sill on the left hand side, this pic shows the seat belt mounting behind the drivers side

I made a repair panel for both sides 65 mm x 15 mm with 45° angle. Reinforced with a 2mm squared tube of 20×40 mm

Painting with anti corrosion prior to welding.   

Tube placed in the middle. Cross section.

 

Final result after welding.

Some fine cutting for better fit required.

Here a cross pic of the panel welded in, it goues right through till the inner back wheel fender

Protected and sealed with wachs.

Instead of the commercial angled nut holder, I made a 5 mm plate, drilles the two holes, welded the nuts and then the plate to the tube.

The inner mounting section I have bought from Robey, They only come in a pair of inner and outer mounts.

The rest is business as usual