Stretching is a waste of time..you'll see a change and then it snaps right back. You need to buy a few sets of valves, and retainers...start measuring and swapping valves/retainers and this will get your installed height where you want it..that's for the shimming part of the questin..but stretching part is for pressure..and if you are having a pressure issue or not meeting the 10.8 rule then buy some more springs or different type as streching won't help.
Yep..a couple years back before there was an .815 , when the rule was only 10.8 @ .850 I shimmed mine..modified 5/8 flat washer=.060..too bad the springs weren't as good then as they are now.