The classic stock is hard to shoot off of bags. On mine I use a very narrow front bag and use the adjustable ears on the benchr rest to make the bag fit the forearm as snug as possible without binding, this helps keep the rifle from canting. I also use a target with a pattern of horizontal and vertical stripes on it so that I can be sure that I hold the scope level each shot.
You did not say what type of group you had, is it a horizontal group, vertical group, or random dispersion? If you supply this information it may help analyze your problem.
Are you using wind flags, is your bench solid, is your benchrest solid, are you muscling the rifle when you shoot it, is your shoulder and cheek pressure consistent? Have you tried different powder and bullet combinations? I have one rifle in 222 that only likes Reloader 7, the groups uising it are amazingly small, with any other powder I cannot get it group into less than 1/2 inch. There are many small things which make the difference between a sub-one half inch group and a 7/8 inch group rifle and many of them are not the rifles fault.
Do not take this disparaginly but if the original barrel did not shoot as well as you wanted and this barrel does not shoot as well as you feel it should perhaps you should have someone else give it a try to see what they can do with it, preferably someone who has shooting abilities that you trust and can rely on.
As mentioned earlier you have pretty much given up any bitching rights to Cooper since you had it rebarreled.
As far as the gunsmith "stretching" the action - who knows what that may mean. If the action truly was "stretched" I don't know what could have occured to cause it or how it could possible be repaired.