RE: sunken mast track
Do you have a file (rather than sandpaper)?
Best way to fit that fin is to rub soap on the fin root (yes, a bar of common bath tub soap like you take a shower with).
When you rub the soap on the fin root, it will be dull and whitish looking.
Put the fin root in the fin box, and put a little hand pressure on the fin.
When you pull it back out, you will see spots where the dull whitish soap has turned shiny and black (the underlying color of the fin).
Use a file to take these "high spots" off the fin root and your fin will drop into place very quickly.
Remember to leave the fin root about 1/64" proud of the bottom of your board (don't keep filing until it comes out flat with the bottom of the board).
This last little bit is the "draw" on the front and rear tapers of the fin root and fin box that makes a good tight fit when you pull the root into full engagement with the fin screws.
If it seems like the fin is not fitting on the lower half of the rear of the fin root (the half closest to the bottom of the board) take a flashlight and look into the fin box and see if there's a "glob" of epoxy there to keep the fin higher in the box.
Cobra sometimes does this, and it's then better to take a very small half round or rat tail file and file off the high spot that the glob presents to see if you can get more of the front and rear tapers to fit tightly.
Hope this helps,