Jbot.. I know they suck.. lol but I want to make it good..
so turn the adapter out of some 4140 or whatever I have laying around. Maybe .005 oversize to the gy6 pulley bore, and leave the taper undersized to the hobbit shaft. Then chuck the gy6 pulley, skim the id to accept the adapter.. press it in, re chuck the pulley with the adapter pressed in it, indicate it in then run a finish pass on the taper to final size..
Might be over kill..
But I know what you are saying, I think this way I have a chace of getting it to run as true as possible..