First welcome to the site - hope you find the information and help here useful in your concours related endeavors
And thanks for including when and where your car was built
You'll learn quickly there are a ton of prior threads and some include information that may be of help. If the threads are somewhat old it doesn't hurt to ask if there is something new, a better product or and update on details or a process but its a god place to start
Just some of the threads already out there '
Rear Spring finishing http://www.concoursmustang.com/forum/index.php?topic=11733.msg70823#msg70823
http://www.concoursmustang.com/forum/index.php?topic=5890.msg33184#msg33184
http://www.concoursmustang.com/forum/index.php?topic=226.msg2036#msg2036Rearend Finishing Really two subjects or even more but the two larger sections/areas are the third member housing and the rear end housing
Rear end housing was painted semi-gloss black before any parts were added. The face, where the 3rd member would attach was bare steel as was the face/flange where the backing plates would attach.
The third member housing and pinion retainer epoxy colored red oxide (so not flat in finish) applied before machining. 9" will have three "half moon" areas on the lower housing and sections on the face of the pinion retainer that were machined so they should look fairly freshly machined and lack any paint in those areas. 8" rearends are slightly different - you didn't include what engine or rearend the car has so don't want to go any more in one direction since it could be incorrect for your application
Same as this discussion - of a 9" application
http://www.concoursmustang.com/forum/index.php?topic=7140.msg40343#msg40343