I did it to my Dyna-Tech. Threaded rod, two big washers, two nuts.
Slowly wind the nuts out to the new setting. The cold setting is only partial - the spring of the steel does the rest of it. Check that you can get a brake to work with a 700c wheel in the frame before you start.
A frame that old might not index very nicely - cable routing and b/b stiffness are important to avoid ghost shifts.