Sorry, but it just doesn't make sense to me that just adding disc brakes also added bump steer. Did you change spindles or steering arms along with adding the disc brakes? Most disc brake setups I've seen simply add a bracket to hold the caliper to the drum brake spindle. The only thing I know of that produces bump steer is by changing the tie rods and suspension pivots to a different angles then they were originally designed to do, don't see how a disc brake setup would change those angles.
Please educate me. Gene