Outdated logic, AKA old wive's tale.

Modern materials, like anything built since I was an adult, really requires no break in. Run synthetic. Enjoy better lubrication.

 

The only explanations I can come up with are 1) lack of oil to the underside of the piston from low oil pressure or plugged pick up screen. 2) the piston was put in the cylinder the wrong direction.

Bill has been doing this for many years and the 351 has been proven reliable. That said you do have me a bit concern since I just installed my 351.

 

I too have these marks on the front and back of the cylinder where the piston skirt rubs. Bill told me this was a normal wear point. By any chance, did you have any issues with the oil control ring being of an incorrect size when you got the kit? I have kit number 532. I got mine late March 2016. When did you get yours/which number kit do you have? I'm curious if there has been a micro change in the specs of the kit causing this effect. But like you, everything seems to run okay like this.
Interestingly, I recently had to get a new set of rings. I installed the oil control ring wrong (snipped it). This is how I found the same markings you have. When I got the new rings, Bill had me measure the piston because he uses 2 different sized pistons for the 351 kit and wanted to supply me with the correct fitting ones. I am now second guessing whether or not i have a correct matching set of piston and cylinder with the proper clearance. I am going to be checking on this