I would make a quick check of the mating threads first.

If you have a fairly decent bench vice (including solidly mounted and jaws square edged), you can clamp the cup in the vice jaws and turn the bicycle frame, keeping everything square as you turn. Depending on how much strength you have, I would not go berserk on the tightening if you use this method.

And only hold the frame at it's strongest points ( like bottom bracket and headtube not e.g. chain stays )

(p.s. I'm an Aussie, hence "vice". I think Americans use vise.)