The answer to the first is
1. Are inner fenders the same from 47-54 - Yes they are.
2. Can't say for sure if GMC fenders can swap for use on a 54. The key change on the 54 fenders is the inside corner of the fender is rounded. If the 54 fenders are rounded in that area then they can probably be made to work with a lit finesse. You can always transplant the corner from your 54 fenders to the GMC fenders to make them work.
Good luck.
