I've done quite a bit of restoration work. Best way on a plank build, do the stock, and get rid of the excess wood to see what you are actually dealing with. Cut and thin shims to fit down into the crack at least a quarter inch. Use Titebond II to glue it in place, gently tapping to act as clamping. From there you can carefully trim it down closer to surface, and rough sand the patch to level to the wood. Excess glue will help fill and voids, that may be there. You shouldn't have any if the patch is properly done. Alternate method is to let the glue set up, then rasp or sand down to surface. If done well, the crack pretty well disappears.