I would check the wall thickness of the pole to the pole vent. The last t2 motor I had, I remember the pole vent being quite large and I'm not sure you would have much metal left on the pole if you took it down to 3". The amount of metal in the pole has a significant effect on BL, so IMO I wouldn't do it if it would result in any less than .5"+ wall thickness.
To do this, you would also need to make a new top plate as the 4" one would not work.
Honestly, it would be a lot easier to machine your own steel set out of new stock rather than try and modify the T2 stuff.
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