That'll be a lot easier job because you wont have to spend so much time fairing. After a board dropped down while I was driving, I had to add 10 inches to a centreboard on my old Stingray which carried from memory 225ft of sail and I was able to stand on the tip for righting after repairs.
I used divinacell as my core because it was easy to shape, I ground back the edge I was joining a long way at an angle about half an inch then I put thick plastic on the workbench laid the board on it, pulled the plastic up to a bit bigger approximate shape. Filled it with wessystem and microfibles & cloth. Turn it over do the same on the other side. Then sand and fair it as you like bit of flowcoat jobs done.
regards