If I understand your question, you want to be able to use the space bar to preview items in a stacks folder. If that's the case, when you click on the stack, one of the items is:
Click that, then highlight what you want to preview and press the space bar.
How is Command-click faster? What do you Command-click?Command click is faster =)
Command-click on the icon in the Dock to open its containing folder in the Finder. Works for apps, folders, stacks, anything. You can even command-click on an item *in* a stack to open its containing folder in the Finder.