Spindle nose on a Denford Orac cnc lathe

I am just finishing up a retrofit of a deford orac cnc lathe. I found
that I cannot chuck up a 2"dia bar with the standard 3 jaw chuck.
Does any one know what the spindle nose is so I can find a back plate
for a 4.5" 4jaw chuck that I have at hand?

Thanks,
CNCZKY