I saw where you chose the Firebird setup but I wanted to add this for anyone else who may see it and want another option too.
Back in the day, the Jaguar IRS was an really popular option to give disc brakes and independent rear suspension to our trucks. Once I got my '57 in '89, I was hooked on the idea. So, I found one at a junk yard from a '73 XJ6 and removed it from the original housing to expose the components.
I bought a mounting system which included the cross member, radius rods and all the supportive parts as well as a rebuild kit that included U-Joints, rotors and brake pads. When I sent the truck to the body shop 7 years ago to take over my rebuild and do the fab work, here's what they did;
The install is solid with the reinforcement of the frame rails with 1/4" plate but the builder's concern is with the failure point possibilities (4 U-Joints on inner and outer axles) and setting it up due to shimming of the system and the time need to get it riding right. I've got about $3000 invested with the fabwork, IRS and build kit but I'm confident it will be worth it as it has a killer look and should ride great with the Camaro clip up front. If all else fails, I could always go with a straight axle setup like the 10 bolt that was in it or a 9" worse case.
The project has been stagnate for the past 5 years but has recently gotten hot and heavy.
I'm hoping to get it out for it's maiden voyage in the Spring. Cheers. / Ken