The width of the flange where the "nose" of the apron is spot-welded is thinner on the '30 fender than on the '31. So you'll have to weld about a 3/16" strip of sheetmetal the length of the flange. Then you'll have to create the slotted holes that are used to bolt the fender to the front of the '31 splash apron. And you'll have to trim and bend the reinforcing strip at the rear edge of the fender (at the area that bolts to the running board), such that it does a 90 degree bend for about an inch toward the front of the car along that flange you just enlarged. Additionally, the crease in the fender that runs from front to back goes almost to the front edge on a '31 fender and it ends or sort of fades out a couple inches back from the leading edge of the fender on the '30 version. The Judging Stds show this.
I think it's a bit easier to go the other way...to take a '31 fender and make it a '30. I just did a pair on mine and it took a couple hrs of work.