Well, you have a good gauge, and can read it

Regardless, it's not a ploy to make some money off the line hone job - every single time I install main studs or girdles I can measure the distortion in the housing bore. If you take the time to measure it, you can even see changes in the bore geometry by varying the applied torque by even 5 lbs up or down. You can't just measure the vertical and horizontal, you need to measure all the way around the bore, and to a certain extent that mitutoyo gauge is dampening your readings as the sunnen gauges that we have I have compared back to back with the mitutoyo and they are more sensitive to higher order bore geometry distortions due to their different centralizer design. Fact of the matter is that you are still only measuring the distance between two points, and I wish had my books on the computer(pictures) but there's multitudes of drastic out of round and distorted bore scenarios that can be had while your stick gauge still shows an in-spec housing bore. And on top of that it will show you nothing of bore to bore alignment through each saddle of the main line.

It is just like cylinder bores, the reason for plate honing with the fasteners that will be used at the applied torque that will be used, all these factors change the bore geometry. Ask any machinist who does honing, put that plate on or studs in the mains and you can literally feel the stones load and un-load as they cut the distorted housing bore into a round shape.

I'm not going to get into a competency argument, I will simply say that you would be hard pressed to find any engine builder that would say it's ok to install main studs and not follow it with a line hone.

And I have had several blocks brought in over the years with garage assembly jobs by people who thought they could just throw studs in and ended up wiping all their main bearings and trashing thousands of dollars in components just to save $350.