8mm thread is my guess too.
I agree - I think that the taper is taking a significant amount of drive torque as well as location. This is why I am reluctant to totally dispense with it but I would like to maximise my Z work space as no machine I own has what I'd consider ample (possibly not even sufficient). My current plan is to cut it down to say 20mm and make up the arbor to suit. I may end up making 2 arbors, one 40 taper, the other R8.
I do have an original Bridgeport boring head (no facing) with a 1/2" shank that fits in a collet and this has been satisfactorily rigid, but using the term 'rigid' has very different meanings when one considers a turret mill and a jig borer (or Deckel style toolroom mill). So I am reluctant to just cut off the tapered section in toto and rely on a collet, though this would be simplest.
I see what you mean about your spindle nose. No removing those dogs. I just double-checked mine and fortunately, all 4 of the 40 taper spindle noses have the drive dogs fixed with cap screws. One less problem. There are ample others.
PDW