I'd rather an ML7 owner explained but in outline:
1. The ML7 has a fixed speed motor
2. Three main Spindle speeds are varied by moving belts between pulleys; they're under the covers.
3. The number of speeds is doubled by engaging backgear, giving six speeds between 640 and 32rpm
lathes.co.uk have lots of words and pictures.
I agree with Old Mart about Brake disks. Disks up to 7" diameter may be possible and thin disks could be stuck on a face-plate, but not ideal.
Much depends on how many and how often. Better to use a more suitable machine if there are lots of them, squeezing loads of big work on to a small lathe is a bind, even if it's possible.
Dave