That is a good way, before put the stab and the fuselage together, to build into the stab one so-called "T-rib", made of 5-ply 2 mm (1/12" or 1/16") thick plywood. You can make this "T-rib" very precisely to 90°. (naturally, it must have a hole in the vertical part for the pushrod).
After gluing the wing, you need to glue first the stab, between the fuselage sides, precisely adjust it to parallel, horizontally and vertically. The rear 1-2 fuselage ribs go in after this step.
Now for you it is better to disassemble the stab, instead to try to turn the fuselage: it won't be never precise and stable, I think...
Itvan