Stack up the form drawings and make the opening where the most wood is around the hole in you worst case (station).

Strongbcks are easy to make. It's a lot of work what you're doing and there's room for error. Why? If it's one of Nick boats - fine do it that way. But if it's someone's design where they use a strong back why not just do that. If your boat is low volume or has a lot of rocker, you may have to stair-step the internal beam or it may come out the bottom of the stern area before it makes it to the last form.

I use a (T) beam strong back. (2 )16 ft 2 x 4's twist nailed and glued into a T-beam. Your cost - about $12. Time about 1/2 hr. Mine is still true and has made 4 boats. Besides the strong back doesn't even have to be perfectly straight - just stable - use dry wood and seal it.