Tuning with jets, Power valves and an O2 guage

Last weekend I drove the car on a 950 mile round trip from Northern CA to L.A. for the Spring Fling. The car ran great and had no problems. The A/F was reading between 11.9 to 13.0 at a 65-70 mph cruise. It leaned out a bit when I tipped the throttle at that speed then settled back to 13.0 at cruise.
At home i figured I'd try leaning out the primary circuit so I pulled the #83 jets and put in #81s. The car ran a bit leaner in the 12.4 to 13.8 range but power dropped off and it actually detonated a little on one hard WOT run. I put the #83s back in and readjusted the idle mixture screws again. The power came back and no knock either.
This engine must like the richer mix. I'm leaving it that way for now.
Thanks, Greg