From Columbia to Dorrington it is around 60 miles. Italian Bar comes out above Twain Harte. On a FS map you will see fs n0l that runs parallel to Italian bar east of Juniper. It runs on a ridge above the south fork of the Stanislaus. Both those are basically just gravel roads but the connectors rds between them can vary in difficulty. To get to Beardsley turn off at Spring Gap. The only pavement is a section of the road to Beardsley from 108 and coming out at Dorrington.
I can make it easy or more difficult, depending on your abilities and what you are riding.
Crossing the Dam is easy, going Hells half acre around the east end of the reservoir is more difficult and adds miles. Italian Bar is easy, taking the Grant Ridge cutoff is difficult, but I did it on my f800 last week.
To come out at Avery requires taking a road the skirts big Trees park. The rd is on Sierra Pacific land and was donated to the park last month, so may be blocked off.
You guys have picked the busiest weekend of the year for campers so expect some traffic, but this route skirts most of the campgrounds.
Normally I am a middle of the week rider, but will be happy to share what I know. I live in Columbia, so just as soon meet up with you up here.
I am the old guy on the 800 that rode Sonora Pass with you last year.