LGT LINES UP WITH WRONG RWY AT ORD; TWR CANCELS TKOF CLRNC.

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WE PUSHED BACK AT THE SW CARGO AREA AND STARTED ENGS. A DC-8 'HEAVY' HAD ALREADY PUSHED OUT IN FRONT OF US AND WAS STARTING HIS ENGS. AFTER WAITING AWHILE FOR HIM TO TAXI OUT; WE HEARD GND CTL CLR HIM TO RWY 27L VIA TXWYS T-12; T AND S. GND CTL CLRED US TO THE SAME RWY VIA TXWYS T-12; F AND M. WE WEIGHED 160000 LBS ON BLOCKOUT AND FIGURED THAT SINCE THE GND CTLRS CLRNC WAS 'TAXI TO RWY 27L VIA T-12; FOX AND MIKE; '#1;' WE HURRIED TO BE #1 FOR TKOF. THE THOUGHT OF TAKING OFF BEHIND A DC-8 'HEAVY' AT 160000 LBS MADE US RUSH DURING THE TAXI PHASE. IN OUR RUSH; WE PASSED UP THE ENTRANCE TO RWY 27L AND ACCIDENTLY PULLED INTO POS ON RWY 22L. THE TWR CLRED US FOR TKOF AND JUST AS WE WERE ADVANCING THE THROTTLES; THE TWR CANCELED OUR TKOF CLRNC AND ADVISED US THAT WE WERE ON THE WRONG RWY. THEY TOLD US TO TAXI FORWARD; TAKE THE REVERSE HIGH SPD TXWY AND TAXI TO RWY 27L. AT THE TIME ORD WAS USING RWY 27L FOR TKOF AND RWY 14R FOR LNDGS. THERE WAS NO CONFLICT IN TFC AND NO EVASIVE ACTIONS WERE NECESSARY. IN MY OPINION; 2 THINGS CONTRIBUTED TO THIS RWY EXCURSION. THE FIRST WAS RUSHING TO BEAT THE DC-8 'HEAVY' TO THE RWY. A DELAY ON TKOF WOULD HAVE SUFFICED TO AVOID WAKE TURB. SECOND; COMPANY PRACTICES OF HAVING THE PF SET HIS HDG BUG ON RWY HDG IN CASE OF ENG FAILURE WAS NOT FOLLOWED. INSTEAD BOTH PLTS PUT 290 DEGS ON THEIR HDG BUGS SINCE THAT WAS THE INITIAL HDG ASSIGNED BY ATC AFTER DEP. THIS WAS NOT INTENTIONAL AND WILL REMIND ME THE NEXT TIME I THINK THAT I SHOULD RUSH. COMPANY PROCS WILL BE FOLLOWED TO THE LETTER FROM NOW ON AND THERE WILL BE NO RUSHING IN MY COCKPIT.