Mixture seems to lean out while cruising but not acceleration/decel and idle

[66StangGuy]

check your pcv valve connections and hoses. i had the same problem in my 66. i added the o2 sensor and gauge..same one you have. anyway turned out the hose was leaking next to the carb and under steady vaccume it would leak and cause a lean condition. identical to what problems you had. take it completely apart and check for vaccume leaks and any bad clamps letting air in.