Well Maybe......
All 30-31 fenders are basically the same... but...
The 30 fenders may not have the same type of lip where the nose fits in place and there are no holes as the 30 nose was spot welded in place.
Some 30 fenders will have a rolled over edge along the frame.
There are some variations in the 31 fenders. The crease that flows from the front of the fender down into the flat section leading into the running boards has 2 variations. One goes further down and has a bit more crown as it approaches the running boards.
There are also some variations in the area where the nose piece fits in, but that is not critical. If you see enough fenders you will see some differences.
Oh, BTW, there is a known original untouched 31 car that has the 2 different fenders.
There are all sorts of different blacks out there. Some have a yellow base, some have a blue base. The off the shelf DCC black is a yellow base. You have to have the deeper blue based black mixed up in DCC. Probably does not matter which one you use, just be sure you use the same type for all your black painting.
When setting up your fenders there are 2 details that are tough to get right, but help set the fenders up as looking good.
Make the crease look right. The crease gets lost because of damage and people trying to do bodywork wrong. Then you lay in too much filler.
Get the bead straight and square. Again this can be lost as people try to get the fender to work.
Both the crease and the bead can be very difficult to make look perfect or even decent. So I can understand why you frequently see these features not very good on the cars.
Here are my hints for fender fixing.
Front Fenders
