I assume you are talking about the version under 'model 0' on the Lathes site.
**LINK**
same seller has a couple of other BT35 items eg
**LINK**
The above would allow you to then use a MT3 arbor (as used I think on the Centec).
You can make a perfectly servicable taper for amateur use from mild steel without flange and put a screw/pin in from the side to take the drive. It won't matter wheter it is int 30, int 40, or int 35 as the extra will just stick out. Then finish bore it in situ for a plain shank endmill and you are in business.
If you can't get the 2.5in bar because the world has gone metric get the next smallest metric and a bit of next biggest (or 'hollow bar'
to make sleeves for the bits that need to fit. It will be much easier to turn a short bit to the correct diameter.
I'm rather puzzled by the date on this thread – I'm certain I replied to a version of it a month ago.