Just bought a 12 bolt for a 66 all looks good so far except one backing plate is bent inward about 3/8" at one point on the circumference of the outermost point. This gradually reduces where it should be so that 2/3rd of the circle is OK. Must have been dropped at one time. You can even see that it is about 1/32 to 2/32" of an inch away from the plate on the axel at a point in the middle of 2 of the 4 mounting bolts, even though it is tight at the mounting bolts and the other 3 sides of the mount plate.

Should I replace?

What is all involved? Have done a search and found one article. But a little unclear about the removal the the pin and or pinion shaft. Is it easy to mess up?

This should be a pretty easy repair.Spin the gear till the pin is near the lower of the right side bolts that holds the differential in.I use a wrench on one of the 12 bolts that hold the ring gear to keep it from turning while I break loose the small pin bolt.
Once the pin is out,the cross shaft will just slide right out towards you.You may have to use your finger to push from the other side to get it out.
Once that is out, just push in slightly at the axle flange and the C-clip will drop right off the axle end.You can now pull the axle out and replace the backing plate.While the axles are out do not let the diff spin as this will allow the side gears to fall out of place.
Don't forget to check the outer bearing and axle surface for wear.You may want to replace the bearings as a maintenence thing.At the very least replace both sides outer seals.I use a peice of medium emory cloth to polish the area where the bearing and seal rides.
Also if the small pin breaks don't panic or try to force anything there are a few tricks for this.
HTH