Hi David.
Alfred is not responding at the moment. I think shortening the tube and cutting new threads is a good idea but I think not many people (and machine shops) will have the equipment to do this.
In fact either method will work ok.
Just harping back about the draw tube possibly vignetting the light path (if the focuser is moved fwd)....hmm maybe not as the older focuser was already 24mm further inwards. So it should be ok?
edit:
Just putting in some more pics from Alfreds CF OTA and focuser and a test of his 5D Mk11 full frame camera with a 1.25" and 2" T ring adaptor.
As you can see the 1.25" vignettes and the 2" ..just a little...which is probably because of the full frame not focuser draw tube.