I would try chasing the threads and using loc-tite to help hold the new BB in place.

A friend who used to work at a beach-side shop with lots of seized BBs suggests using some sort of impact wrench in order to salvage the BB shell when removing a seized bottom bracket. He said the heat generated from all the torque of slowly spinning it out of the corrosion or rust usually pulls the threads out of the shell, whereas quick bursts of energy from the impact wrench can avoid overheating and stripping.