The last time I put one together I tried putting the outside spring leg in its little plastic bushing before installing the offset arm/cam/pivot bolt/etc. With the offset arm and cam against each other a small wiggle popped the cam over the steel ball in the centered arm and let me finish putting it together.

It was a little different adjusting pre-load with spring tension but seemed to work great and was much easier than fighting the spring after the brake was otherwise back together.

As a foot note those springs eventually fail due to fatigue so having spares on hand is a fine idea.