snobol, 126 bytes

Result

AC

Duration

806ms

Code [DL]

	T = INPUT
F	OUTPUT = T
	DIFFER(DUPL(' ',SIZE(T)),K = INPUT)	:F(F)
	OUTPUT = TRIM(K) DUPL(1,SIZE(TRIM(T)) - SIZE(TRIM(K)))
END

stdin

                                           T      
                                                  
                                                  
                                                  
                                                  
                                  K               

stdout

                                           T      
                                           T      
                                           T      
                                           T      
                                           T      
                                  K111111111

stderr

strace