Which size of vise should I get for a bench mill with a 840x210mm table ?

[OCJohn]

I have a 4" Shars that came with a swivel base. I removed the swivel base and made a backing plate to fit the swivel base with the same 1 1/2 x 8 tpi threads as on my lathe spindle. This allows me to swap a chuck back and forth between the lathe and mill without loosing concentricity on the lathe.

That's a great idea. I got a couple of extra swivel bases in my scrap pile just begging for a new purpose.