The SBC dampers will fit right on the 250 crank whether they are from an internally balanced SBC or from the externally balanced 400 SBC however none of the SBC dampers of either type have any belt pulley grooves machined into the outer ring.
If the damper you have on the engine now has 1 or 2 belt grooves in it then it is correct for the inline and the external balance problem is not a factor. If it is the right damper for the 250 it still may have a balance defect but it normally wouldn't be as severe as if it was the 400 SBC externally balanced damper.
Another question is did the original damper fly off before or after you had the crank work done?
Knowing this might help focus on the problem you are having now.
Mike G
