I have been using the same Borla header/muffler manufactured in 1994. The reason the tab(s) break off either the header or muffler it from the twisting of the header pipers inside of the muffler. The header touches the chassis, the front of the chassis has a ton of flex and moves the header around. If you don't beleive me, next time you have your header off, look at where it rubs on the chassis.
The fix is to leave the bolt loose allowing the tabs to float. I use a longer bolt, leave an 1/8" gap, double nut it together and let everything float. The tab(s) will not break off.
Randy - RPM