Would have thought 11-12 mm dia would be adequate over that length.

This might be a daft idea, but unless you want to bore all the bearings at exactly the same diameter (ie without moving the cutter bit) could you have several cross holes in a shorter bar and move the cutter along.

You could also use a bar nearly up to the drill size but file flats where the tool pops out to leave some space for chip clearance

Holding one end of the boring bar in a 4 jaw rather than between centres makes the whole setup stiffer as well, just don't take the boring bar out till you've finished.

My advice would be to ensure that all the Main Bearing parent bores are cut in one operation, (This has been the practice in Industry for MANY years ) with one tool. In this way, they will be in line, and the same size.

Setting multiple tools to the same size will be difficult, so why introduce extra complication and risk, needlessly?

As Jason says, make the boring bar as large a diameter, and as short, as is practical. This will maximise stiffness, (less tendency to whip/deflect ) and accuracy, as well as reducing the risk of chatter. Rough out with a drill, and finish to size with a stiff Boring Bar. (If need be, set it up on consumable material, ready for use after the roughing operation )

Howard