I believe I read somewhere that thicker diameter will be stronger (less flex) than thicker wall. (i.e.: 3/8" x .080" wall is stronger than 5/16" .105" wall.)
Most related parts are designed for 5/16" & larger. Smaller diameter pushrod ball ends would also concentrate more pressure on lifter and rocker arms. Mopar OEM pushrods have been known to punch through stamped non-adjustable rockers when pushed past their limits.