right and that's the first step, IF you can find somebody with the thread die who can be trusted to extend the existing threads AND knows how.
I've had no luck finding a local shop willing to use their die, (apparently some tools are now only for show, or the last qualified mechanic has left their building) plus more than one shop did it but with such horrific results I wish I hadn't found them.

Next step is to have one of those Park SG-1 saw guides (or the more $$ SG-6 or 7 models) and a nice sharp hacksaw blade.
Easy-peasy, but might have to also extend the slot for washer tab