ACR MLG WRONG RWY APCH AT ATL.

Narrative

DURING APCH TO ATL; APCH CTLR; ON CHKIN; SAID 'EXPECT 27L'. PF BRIEFED AND WE TUNED/IDENTED 27L APCH. I ACKNOWLEDGED FINAL INTERCEPT TURN TO 240 DEG AND 5000 FT; PARTIALLY BLOCKING CTLR'S LATER INSTRUCTIONS. I QUERIED CTLR WHO SAID 'CLRED FOR APCH; MAINTAIN 2700 FT TILL CATTA'. I REPLIED '2700 FT AND CLRED FOR 27L'. NO RESPONSE. WE HAD BEEN GIVEN 119.5 FOR TWR (N TWR). CHKED IN WITH TWR AND WAS SENT TO 119.1 (S TWR). CHKED IN AND TWR SAID 'THIS WON'T WORK; YOU'RE 3 MI BEHIND HVY; TURN L TO XYZ DEG AND CLB TO 3500 FT'. ON CHKIN WITH APCH HE SAID 'DID YOU SHOOT APCH TO 27L?; YOU SHOULD HAVE BEEN ON 26R'. REENTERED FINAL FOR UNEVENTFUL APCH. FACTORS: 26L WAS NEVER MENTIONED; AND WE FOCUSED ON 27L APCH. I SHOULD HAVE INSISTED ON READBACK OF MY 'CLRED APCH 27L'. THE TWR FREQ 119.5 (N) AND 2700 TILL CATTA SHOULD HAVE ALERTED ME TO CONFLICT; BUT WE WERE BUSY AND STILL FOCUSED ON 27L APCH. FREQ CONGESTION AND LENGTH OF INSTRUCTIONS CONTRIBUTE TO THIS PROBLEM. (DATA LINK WILL SOMEDAY HELP.) INSISTENCE ON POSITIVE READBACKS IS IMPERATIVE FOR FINAL CONFIRMATION OF INSTRUCTIONS.