As I recall, all that was needed was a small notch out of the bulkhead. The compression post (the existing and new one i installed) did not need a notch for the wiring. The compression post has a dado (?) groove running its entire length and the bulkhead slips into this notch. At the top, the bulkhead has a small piece missing to allow the wiring to snake through a hole in the mast step and down the side of the compression post, hidden behind a piece of teak furring.
Incidentally, where the mast light a radio cables penetrate the step leaves a vulnerable location for water to get in. The mast produces condensation inside even if water does not get in they any other way, and this water collects in the mast step. There were two (small) weeping holes in the mast step that were supposed to let the water run onto the deck out the side of the mast step collar but the water could still go down the wiring hole and into the cabin, and indeed it had been. I welded a small pipe nipple over the hole in the mast step where the wiring goes through so that water cannot go through the wiring hole and into the cabin. I also drilled out the weeping holes a bit to let the water out easier. All of this was at the advice of a very wise boat building who was helping me with my overhaul. I would not have thought of that myself.