Now that I bored out my chuck to 3/4" I can fit 5/8" hex bar too, so I made the boring bar holder I've been planning:
Face and center drill the 5/8" steel hex bar,
Drill out (5/32, then 5/16)
(the 5/16 drill ended up almost 11/32 with a new Chicago Latrobe bit!)
Ream to 3/8",
A little counter-sink,
Cut to about 2", face and countersink the other side,
Getting there,
Cut a slit to allow clamping screws on tool post to tighten the block,
A quick hit on the buffing wheel to make it shine,
Ready for my next adventure in boring
(Now I need to start a new project to try it out!)