Position of a Flat in a Key

Hi Guys,

at the moment I am working on the transcription of a manuscript form early 1700. I try to stay as close as possible to the manuscript and that's why I am notating on 6 lines.
But now I noticed that if I want to use an F-major key, the flat of the b-flat is positioned on the high b-flat in bass system. Does anyone know how I can change to the lower b-flat as it is notated in the manuscript?

Thanks for your help!