Old school boring bars bolt to the deck of the block using that as the reference plane... Plus they are designed to bore maybe a 12 inch max travel...
The main bearing saddle doesn't have a machined plane to bolt to at either end of the block.... Timing chain cavity at the front & flywheel cavity at the rear.... What are you going to rig as your reference plane...
Then realize the only way your going to bore the full length of the main saddle with a machine with a 12" travel is to work from both ends.... Talk about a recipe for disaster...