You will not need to use any sealer if you are using a good quality head gasket. If you are using a poor quality gasket then you could use a sealer such as Copper Coat but it is not going to make a poor quality gasket perform as a topline head gasket will. If a sealer is used then careful care...
One dimension that you have not mentioned and could cause the steering pull issue is the diagonal center to center. If you place the car on four jack stands and set them as close as possible to the same distance from the hub face as possible drop a point to the floor directly from the center of...