I've just done this to my son's GT, it's a cro-mo frame, the bracket fits on the end of the tube (where it's thickest) and the dropout. I'm using a 140 disc to keep the braking forces lower, but figure they can't be any worse than the loads fed into the middle of the seatstay from canti's.
I made the tab myself and TIG'd it on, all it's cost me is fresh powdercoat, some new decals and a bit of time, but them i'm lucky to have all the equipment.