We've dealt with this in the past by using bolts long enough to pass through the saddle bracket, all the way up through the box section. Of course, you'd have to drill the top of the box section out to make this happen. [Actually, this is how we install oversized bars on many of the old Volvos that have the earlier, one-bolt, arrangement.]

If you do this, please use large washers above and below, and be sure to not compress the box section itself. That would be bad.

Ron's approach should certainly work well - but if you don't have a welder handy AND the surrounding material isn't torn up (that is, it's just that the nut is absent), it might be simpler to just drill another hole and use another bolt. And washers.

Unlike some of the advice you've received on the 1800 yahoo list, I believe that the box section is stout enough to hold onto the swaybar -- IF it isn't rusted... but it sounds like yours does suffer from some corrosion. Several of the cars we've done with the longer bolt and big washer method have seen aggressive use, both on the street and on the track; so far, no evidence of fatigue and no failures.

Again, this assumes there isn't any rust that's compromised the structure of the box section itself.

Best,

Cameron
Rose City