One method I saw on U-Tube was to bolt the block to the cross slide. You need to make up a boring bar with bushes on the ends, which fit the block. The boring bar is then driven, via two universal joints, by the chuck. It's not that difficult, because the holes in the block doesn't need to be at centre height. The saddle is then moved along to bore the holes.
Thanks Stackerjack. Rarely is this method proposed on threads like this.
This is mathematically and technically the correct way to make a parallel bore. Simply, you are describing a circle with the tool and progressing the circle along a straight line of the axis of the bore. The way it is always done in industrial manufacturing.
Personally, I simply mount the block where the tool post would normally fit, and use a boring bar -central tool - mounted between chuck and revolving tailstock.
Or I mount a boring bar (end cutter) in my miller-driller and progress the cut vertically down through the engine block, which is mounted on the cross table. Easy!