I recently rebuilt a blower snout, which was not too hard. I haven't torn into the rotor pack, but I think you could time the rotors if you did.
Disclaimer: This is all untested ...
My idea would be to drill two holes in a plate that are precisely located to fit the rotor pins. (Alternatively, maybe you could use a cut-up blower case for this fixture.) Then you could use feeler gauge stock (those super-thin strips of metal) to shim between the rotors where they meet and make sure that the clearance is the same on both sides. Then you could lock the rotors in place (maybe using a dab of some glue that you could remove after you are done). Then line up the gears in a similar way (using shim stock again if necessary) and press them on.
One other word for the new fella: Rotor packs from recent GM cars equipped with M90 superchargers are interchangeable with these rotor packs. They have two key benefits: first, they are much newer; and second, they are coated with an epoxy coating which increases efficiency (an improved version of the coating on the 1994-95 Thunderbird SC blowers). All you have to do is drill and relocate some locating pins. So, this could be a good "Plan B" for you, if not "Plan A".
1990 SC auto ... red outside, gray cloth inside ... ~205,000 miles (and climbing) ... driven daily!