M. de Garsault tells us: "Before sewing the fronts to the backs, pin them together and check by the measurements. Sew the armhole down to where the pleats begin, then sew the shoulder seams..." I'm not sure how this was done in the 18th century. Women were fitted for stays and had their gowns put together on them. Since the lining seams are open, I have pinned them out of the way, and buttoned the coat inside out on my form.
The center back seam looks a bit wonky, so I'll need to take more out of that by pinning and marking it with chalk. Adjustable men's dress forms are hard to find, but vastly superior to stabbing yourself with pins and bleeding on your project.
Here is the mess from the front. I only made the mistake of fitting a coat WITHOUT a shirt and waistcoat on the form once. Perhaps the best part about having a form is it allows you to step back and say "WHAT THE HELL HAPPENED THERE?" And "THAT'S NOT STRAIGHT! I HATE SEWING."
After all the side and shoulder seams are back-stitched, the lining seams are turned under. These are pinned together and whip-stitched, then I can try it on. Next time more grumbling.