You might want to pick up a small length of "top hat" section to form the strengthening section inside the chasssis leg and some hollow tube of the correct diameter for the bush mount. All in 2mm (minimum) mild steel. Best way to get access is to take an electric jigsaw and steel cutting blade(s) to the chassis cover panel in the boot, taking the outside 3 inches of boot floor as well!
The complete main chassis leg is available from most of the Brittish suppliers, if things are as bad as mine were (Chris & Julie D? ..want to comment?). theres some complex compound curves in it.
I'm rebuilding the back edge of the cover panels (where they meet the above-boot back edge crossmember) to be a little further away from the leg itself, as there's a terrible crevice there which is only about 1/4 inch wide, a real magnet for mud and salt.
I only hope your chassis closing piece, and under-tank panels etc etc are in good condition.
sounds to me like you bought a Scottish Alpine!
Andy