Any Old School Machinists Here?

I would suppose you could bore the main caps with a deck mounted boring bar if you are careful.

You‘d have to get the boring head indicated in and perpendicular to the bore axis.

Then you’d have to be careful once you get past the second main because the bar is hanging further out of the housing. Of course, if you leave the mains .004-.005 undersized any misalignment if it’s fairly close can be honed out.

You could do it, but you better be detail oriented or the mainline will be all over the place.