What is the best way to secure (2x6) wood framing to the concrete portion of the garage opening?
Yes, use pressure treated wood. There would often be anchor bolts cast into the wall to hold the 2x6s, these can be reused. Sometimes powder actuated fasteners are used. Besides needing a special tool to install these, they often do not penetrate well into old, well cured concrete.

You can install new expansion anchors by drilling holes with a hammer drill. Whichever method, back it up by using construction adhesive to limit future warpage.