rear axle

I think it will fit, but I'm asking anyway...Can I use a 12 bolt from a 68 Chevelle in my 69 Firebird?  

Aside from possible width issues, you have to consider that the chevelle, like the gto is a coil spring car with triangulated 4-link bars. The firebird uses parallel leaf springs, like a camaro or nova/ventura. That being sais, you can have your leaf spring brackets cut off your 10 bolt and welded onto the chevelle housing by someone who will take the time to make sure they are straight and square as well as cosider the pinion angle (very important, wrong angles lead to vibrations and broken u-joints). The chevelle axle is likely an inch or two wider from backing plate to backing plate, so your wheels will stick out further. Brakes should be interchangeable but check to be sure. A better choice is a 12 bolt from a camaro. The obvious question is why? Are you running huge power and breaking things or just looking for posi/different gears? If it's posi/gears, try later models of firebird/TA/camaro (70-81).The 8.5" 10 bolts are plentiful and bolt right in place of the 8.2" 10 bolt.
Good luck.  
A little help... 'cause we don't all have to learn the hard way!