Welcome to the Bolt!
1st question...Are they interchangeable, allowing me to use the original grill/hood?
Yes, they will bolt to your cab.
Yes, your existing hood will work.
No, your existing 3000 grille will not work without modifications/body work...with enough time and money and anything is possible.
FYI, your core support and inner fenders will work as they are the same on all series trucks...
2nd Question...Where do I find 6000 series fenders?
Post a ad in the Swap Meet
section under Parts Wanted, swap meets and junk yards, they have never been reproduced.
NOTE, 1955.2 & 1956 4000/6000 front fenders are the same. 1957 4000/6000 fenders are different, they have a larger wheel opening due to the front axle being moved forward 2.5". Not sure if this matters to you...