I had one of the 669s, and had a fine trigger job done. (Remember Gunsmith George Stringer, some years ago, who was a regular on the forum?)
I later sold it to the guy who owned the indoor range where I shoot. A year later it developed a crack in the frame, and S&W replaced the gun -- seemingly embarrassed by the crack. (They didn't replace the trigger job, however.)
I don't know that changing the recoil spring will do much to protect the frame -- as the recoil spring isn't primarily intended to protect anything, or even to attenuate recoil -- but simply to close the slide.
(In some guns, a much stronger recoil spring will actually make things worse, as the stronger springs store more of the recoil power and send the slide slamming forward with greater force, and the only thing stopping it is the slide stop...)