I've put 120lbs (muiti-day water load) on a Surly brand rear rack. They are very adjustable. I have a tubus on a second bike.
The three cromoly rack manufacturers I know of are Tubus, OMM, and Surly/Salsa. I wouldn't hesitate to buy any steel rack used.
Tubus tend to be cheaper if you order them from overseas, but they are legendary in bike touring circles.