The first thing I would do is try to determine if this is a crack in the tube or just the finish. One way could be to drip some fluid (WD-40?) inside the tube and then position the frame so the "crack" is at the lowest point. Any fluid seeping through to the outside?

If it is a tube crack then that tube is seriously compromised. While a tube can be replaced on a steel frame once done (including the paint/decals/shipping) the cost can be too great for most to be good with it. When the frame is a valued family item or has some pedigreed by a former owner (team, racer of note) the cost to repair starts to make sense.

Have you tried to look inside the other tubes for rust? BB shell, chain stays, seat tubes all are areas of standing water making rust there more likely. Andy