Repair guy

[olderdan]

Assuming you only need the morse taper adapter for centre work and do not need the mandrel taper for any other fitments the obvious course of action for me would be to alter the adapter to suit.
A Colchester lathe that I used to use had the same type of adaptor and it was relieved on the centre portion with a .75 inch land at each end, this is common practice on Jones & Shipman cyl grinders also which leads me to think you may have a shop made adapter.
You could do this to yours using a turned service morse taper for mounting and then stone and lap the small end to a correct fit, much easier than trying to grind the mandrel bore.
I always go the simple route if possible.
Hope this helps