Well since Im in the process of figuring that out Ill tell you what Ive been told and what I think. Basically boring out .020 (assuming its done right)guarantees that the cylinders aren't warped or have irregular wear, and assures a tight fit for minimal blow by. Its the best thing to do if you have the means of doing it, and more important if you have high mileage. I have found custom forged pistons (in stock) at stock bore for the 420A Chrysler with a compression of 8:5:1 for $379. Not quite exactly HRC/JE piston's ratio of 8:8:1. I've talked to my mechanic (owner of Diamond Star Automotive) and he said with my 30k mileage I shouldnt worry about stock bore pistons giving me problems. I guess Ill find out :-)
Here is where I found the stock bore pistons:
--http://www.raceeng.com/importengine-main.asp
Hope I helped, lataz.