Tried fitting the vertical stab tonight. Frustrating.
Because the horizontal stab is all the way forward against the aft edge of the turtledeck skins, the forward spar section of the vertical stab could not fit in front of the horizontal stab as per design. Tried "persuading" it a bit here and there, but no luck.
Came inside and researched some other builder websites; contacted a few other builders, and it turns out they had to notch the aft edge of the turtledeck skins (see marks in pic 2 below). Marked the spots for initial trimming.
Also checked the up-elevator clearance, as other builders noted that the elevator control horn could interfere with the vertical stab - it does. Just a few more fit checks.