You really do not have to change out the vari drive unless you just want to. You can use a cheaper commercial flange mount 3phase and add a shaft adaptor to the motor shaft to replicate the original BP motorshaft. Works fine here. AND much cheaper.

You're absoluty right, and as a CNC turner it would have been no problem for me.  However, there's a number of other factors that come in to play.  Courtesy of planning regulations, my workshop roof isn't as high as I'd have liked, and I'm actually turning the new motor upside down from normal, and hanging it over the side.  I can't actually get the drawbar in and out without rotating the head - not a huge problem.  There's also the fact that we're now paying about £100+VAT for the drive belts, and they just ain't lasting like they used to.  I've checked all the pulley faces and the bearings and everything else, but the belts are only last a year or so.  Thats far too much of an outlay for me every year.  By swapping to A section belt, its only ten quid for a new belt, which going by my drill and lathe should last 20 years or more!  The material for the pulleys is only costing about £40, so I'm well below the £100+ for the new belt - which was also needed by the way, when the motor went it took the belt with it!

You can also do it quite simply by using a reflective optical pickup on the timing belt.  Just paint a section of the belt white.

I've never had much luck with reflective pickups, for me they just don't work, but you're right, it is a good answer.

When I had the old step pulley Bridgeport I just cut a slot in the top pulley as I never used that ratio with the VFD. I then used an opto, Optek OPB917b.
If you are going to be making the pulleys then it should be fairly easy just to leave a bit extra meat on the top then shove in a grooving tool deep enough for the opto to fit in and then just slot out a bit about 10mm wide on the mill to trigger the opto.
Hood

Thats an excellent idea...I might just go with that.  I was pondering how to hold the pulleys while leaving enough metal and turn it off again when second ending.  Alternative, I could just bolt a disk to the top.

Thanks for all the input guys, you've given me lots to think about tonight - its too hot to go in the workshop!