How do I flatten the top of the frame rail?

[Gregski]

I am looking for advice on how to flatten that raised section of the top of the front passenger side frame rail. Correct me if I'm wrong, but I believe it is there to accommodate the lower radiator hose. Regardless I am converting my '69 to an electric and have plans to set a battery pack atop the frame in that section so I would like to make it flat like the other side.

I hope these picture convey what I am trying to accomplish.

Thank you in advance,
g