For any holes I had to fill I would cut away all the rust possible, and then once I have a hole, to strengthen the curved pieces I inserted (through much twisting and cursing) some galvanised channel section specially made to fit inside, on both the inside and outside of the curve.
Once I had those welded in place I made a plate to cover the gap between them and plug welded it into place, then sealed the pieces all round by welding it into the chassis.
The new trailing arm sections went in along with some outer parts of the chassis. My welding greatly improved on these parts and I am quite proud of the work on the chassis trailing arm sections (below)
Below is one half of the new segments in place. The trailing arm section (the matt looking piece) and the new outer section (the one glaring due to lighting) are in place and I will begin work on the next side soon.