The factory k frames and frame rail tolerances are very sloppy with these cars. Thats why TTI publishes engine placement specs. Cause if engine not located correctly side to side and height wise the headers wont fit.
These cars are 50++ years old. Were they ever in a accident? How many motor swaps, like the OP here states a BB was in his car using elephant ears. So much to consider, and no way to make a header to compensate for all entire history of the car.