“I'm sure for your purposes it could be salvaged after correcting the taper by grinding the face and the drawbar boss back to the correct distance from the gauge line on the taper“
Where is the gauge line on the taper? Top of flange? At the “break” where the taper begins above the flange? That’s what I still don’t know.
As much as I hate to suggest it, that information can be found in the Machinery's Handbook.
Alternatively a google image search should throw up plenty of results.
For reference, the gauge line on a steep taper holder is some short distance from the top of the flange. There is a specific diameter at the gauge line on the taper, and all other geometry is referenced from there.