I removed a damaged down tube from a BB shell in the following way:

1) Cut off down tube about 1/2" above the BB shell.
2) Use a hacksaw blade in a holder tool and cut radial slots down into tube until you just hit the braze. About 5 slots will be enough.
3) Flux up the shell
4) Aim the torch flame on the inside surface of your down tube, heating it quickly.
5) Once good and hot use some strong pliers and peel out one one piece at a time.
6) After the shell cools use a sanding drum to dress the shell ID.

All this said, I still say cold set the ST to DT angle, or at least try.