The latest....
Well the backplate fits ok and I managed to drill it to take the chuck retaining bolts and tapped it thru M14*1....
Second phase was to machine a soft end arbor for it...Duly acquired from ArcEuro...I proceeded...
I realise this thread is a POC without photos.....I didnt take any..just as well because the whole job has been one complete nightmare......
Once assembled, the chuck srcewed onto the mores taper adapter has more than an acceptable level of runout

So its scrap...
Plan B is to get a 125 mm backplate for my 9*20 and modify that to take the small chucks.....
Laterz...