SCTs have limited light circle. Using this:
http://timosastro.1g.fi/tools/focalreducer.html
With 6,3 reducer you get 26.92 mm circle, 22.08mm at f3.3. Nikon D3s is full frame CMOS 23.9mm x 36mm sensor so you will get vignetting no matter what
If that Meade has true 2" visualback (2" diameter of the optical path, not just 2" visualback) then you could use things like Antares 2" 0.7x or Antares 2" 0.5x Focal reducers, however - they aren't flatteners, and they have to be screwed quite close to the DSLR sensor.