Just a guess based on the poor job with other machining operations on many factory blocks,they used a old cam core with grooves cut in it to make it usable. The problem would be that every set of cam bearings installed in that block will need "work" if the bores are not correct. I prefer Mikes plan but other methods obviously are doable.

We also hone the cam tunnel like MM for proper babbit cam bearing fitment.... just did 2 R3's today.

Have never had a cam go in "tight" & bearings need scraped, when the tunnel was properly sized before bearing installation.