Propane torch to heat the bottom bracket shell. Since you already have rust under the paint, charring that area isn't a big deal.

The key is to heat the entire BB shell as evenly as you can, without getting any really hot spots concentrated in one area.

Be careful. The paint will smoke as it burns. The grease may also smoke somewhat.

Don't overdo it. The idea is to make the BB shell expand without putting in enough heat to make the BB itself swell.