From the bow to the aftermost
: frame, everything is fine, but from the last frame to the stern,
: I've got so much extra fabric on deck that it looks like it's
: pleated.

I've not sewn up a C12, though I have done so with the C15 - with no issues, and similar shapes. I'll venture to say that it's all in the direction(s) that you pull the fabric as you go along. Not having a pic to go by, I'd suggest unstitching a good bit, then working with the fabric (by pulling hard on it in various directions) to find the best fit all the way to the aft stem. Clamp it all together and start sewing again, pulling it tight as you go. You will, perhaps, have wrinkles still, but hopefully much smaller. Hit them with the iron or heat gun - carefully! - and those should all go away . . . .

Feel free to pm me, preferably with pics, and maybe I can be more helpful.

Good luck!
Dave