dat_A="""file_name
3M CO
ABBOTT LABORATORIES
ABC INC
ALTRIA GROUP INC
AMERICAN ELECTRIC POWER CO"""

dat_B="""crsp_name
A & E PLASTIK PAK INC
A & M FOOD SERVICES INC
A A I CORP
A A IMPORTING INC
A A R CORP
ABBOTT
ABBOTT LABS
ALTRIA
ALTRIA GROUP"""

df_A = pd.read_csv(pd.compat.StringIO(dat_A))
df_B = pd.read_csv(pd.compat.StringIO(dat_B))

crsp = df_B.crsp_name

ratio_threshold = 50

def fn(t1):
    ratios = crsp.apply(lambda t2: fuzz.token_set_ratio(t1, t2))
    iMax = ratios.idxmax()
    rMax = ratios[iMax]
    return crsp.loc[iMax] if rMax > ratio_threshold else f'{iMax}_{rMax}'

df_A['crsp_name'] = df_A.file_name.apply(fn)

file_name              crsp_name
0                       3M CO                   2_46
1         ABBOTT LABORATORIES                 ABBOTT
2                     ABC INC  A & E PLASTIK PAK INC
3            ALTRIA GROUP INC                 ALTRIA
4  AMERICAN ELECTRIC POWER CO                   1_43