Trim the shaft by 3mm. With the actual trimming, what is easiest is having the harness removed from the bass and using a stationary disc sander, but you will have to base this part on what tools you have available. I think a bench grinder works well also. If it’s too much of a pain to remove the preamp at this point then the best thing is a dremel tool with a grinding wheel. Just be sure to masking tape off the entire face of the bass so you dont wind up with aluminum particles all over everything. As far as measuring out and marking out the 3mm that needs trimming, there is a hack. If you happen to have any brass sleeve adapters just drop one on the end of the shaft, what is exposed is 3mm. Grind top shaft until you are flush to the sleeve and you are good to go.