I have a nice Hay Budden 200lb Anvil which has a serial # 200367 on the base.  Would anyone be able to help me out with the date?

According to Anvils in America, Late 1912 - Early 1913.

 

Is the serial number on the bottom of the base? I have a 150 HB in rough shape the waste has been broken and welded back together and one side of the face of it is so rounded over that you can see the separation between the hard face and the body. I’m not sure whether to weld it back up and make it like new or to leave it the way it is and just use the different areas for different things, I know it’s a touchy subject for a lot of people.

You should start a few thread and post photos.

From looking at the pictures you can clearly see the serial number is on the front of the foot (see the yellow clamp).