If it were me, I’d split your block on the bandsaw, if there is decent thickness to it. Then, squeeze the part of the brass you want reduced with small bumps in a vise, constantly easing off and turning the brass. It has worked for me, but I’ve never tried something that big.

I would guess it’s a typo, but check your hole size? If you try this idea, the hole you drill should be over sized and you can bring the two halves close enough together by flat grinding the touching surfaces. Good luck working it out.