AC spindle drive (VFD) on vari-speed B-port head

Hello,
I'm interested in retrofitting an old Bridgeport Series 1 CNC (the
one with stepper motors on it) using servos. I'm also interested in
using a variable frequency drive to control the spindle motor through
software. How do you deal with the variable speed pulleys in the
head when using a VFD? This also applies to my Hardinge HLV toolroom
lathe with a variable pulley system which I would like to put an AC
spindle drive on. Thanks for any help.