It'd have to be 125mm to screw into the quill ?
That'll be it then....

You'll need to mill/file a slot into the block to clear the end of the taper (the draw-screw must pull the taper into the quill - it's not just there to stop it falling out)
Damm you people have good eye sight. Realised that after I made the key....Thought about grinding a bit of the chuck, but dont want to damage it. Theres not much of the quill at that point to support it tho, might have to remove to bevelled edge. This option isnt looking so good now.

Decided to drill for both options just in case the key idea fails. May help someone else who wants to do this but dosnt have a lathe.

Started your drill extension holder tonite. should work fine. Just my poor machining skills letting me down now. Still practice makes perfect, (havent used a lathe for 15 years)