i am thinking of buying a 70-81 camaro 12 bolt rearend. what kind of fabrication would i have to do to make it fit? is it worth it?

 

lots
no

check out moser or strange for a bolt in 12 bolt

the issue, besides width, is the torque arm. No production 12bolt came with the provision and welding to cast iron is not easy or reliable.

 

Second gen has leaf springs, third gen uses coil springs and torque arm suspension. Although the spring, shock, LCA and panhard bar mounts can easily be moved over to a different style diff, there is no easy way to attach the torque arm. That's where swapping the diffs will become difficult and expensive.

In the long run, it will be cheaper to just buy an aftermarket bolt in housing.

 

There's one other tiny little detail that all 3 of you have missed.
The ONLY 2nd gen to have a 12-bolt was the 1970.
So yes there is a ton of fabrication to put any other year in a 3rdgen as a 12-bolt.
The first obstacle being you have to turn it in to a 12bolt.

 

... and ONLY the Z28 and 396ci cars got it.