let see if i can 'splain it better, maybe friend Keith will chime in with corrections
I'd increase the cutback to 8 or 10"
35 degrees was not the stem angles, it was the angle of the sides (away from perpendicular \__/). That pirogue seems to have sides that were almost vertical, well maybe 15 or 20 degrees. Spreading the sides out increases the displacement (footprint of the boat) as the boat is loaded. It also offers more stability as you lean from side to side than those with more vertical sides.
I'd leave the swag alone. The plans call for two 10" sides and a 24" bottom to be cut out of the center a 48" wide piece of plywood. Two 10's and a 24 is only 44" so they're allowing you 2 inches of slop in your cutout. If you can't cut closer to the line than 2 inches, then maybe you need some practice before starting to build a boat.

Lay out the parts a little closer together and make precise cuts and you could an inch or better on the sides.
Three things affect the amount of rake:
1. the swag in the bottom, the less swag the greater the rake
2. the angle of the sides, the greater the angle the greater the rake, and
3. the width of the boat, the greater the width the greater the rake
Just for demonstration, get yourself some posterboard and cut two rectangular pieces scaled to sides that would be 12' long and 1' high. Hold the middle of the pieces vertical to a table and have someone bring the ends together. Vary the three things I just talked about and you'll see how rake varies.
Not sure when Patterson Festival is, if it's in April, we'll likely be there. Not sure what we'll bring. We're working on dugout, a bateau and a skiff right now, but they'll probably be completed by April