Three-time Asian champions Pohang beat Thailand's Chonburi 2-0 to secure a place in Group E alongside 2008 champions Gamba Osaka of Japan, Uzbekistan champions Bunyodkor and play-off winners Adelaide United of Australia, who beat Indonesia's Persipura Jayapura 3-0 on Thursday.
Esteghlal ensured Iran will have three representatives in the Group Stage after posting a 3-1 win over Saudi Arabia's Al Ettifaq to advance to meet Qatar's Al Rayyan, Al Nasr from the UAE and AFC Cup holders Nasaf from Uzbekistan in Group A.
Al Shabab Al Arabi, meanwhile, brushed aside the challenge of Uzbekistan's Neftchi after posting a 3-0 win in Dubai to secure a place in Group D alongside Qatar's Al Gharafa, Al Hilal of Saudi Arabia and Piroozi from Iran.
At the Steelyard, Hwang Jin-sung's free-kick set 2009 AFC Champions League winners Pohang on their way after 28 minutes before Park Sun-ho's 70th minute header secured the win for the 1997 and 1998 Asian Club Championship winners.
"The weather was cold, but we were able to win," said Pohang coach Hwang Sun-hong, who was part of the Pohang squad which won the back-to-back Asian Club Championship titles.
"Chonburi were very fast as we expected and played very well. It was a very good game and we were able to get the right result."
Esteghlal, meanwhile, advanced after goals from French striker Goran Jerkovic, Khosro Heydari and Arash Borhani ensured the Iran Pro League side followed up last week's 2-0 win over Zobahan in the first round of the elimination play-offs to secure a fourth consecutive appearance in the Group Stage, with Yousef Al Salem netting for visitors.
"It was a very stressful match and this was a massive win for us," said Esteghlal coach Parviz Mazloumi.
"We needed to reach the ACL or we would have had to play in the AFC Cup and we belong in the ACL."
Finally, a double from Essa Obaid and an own goal from Aziz Alijonov, who turned a cross from Azizbek Haydarov into his own net in stoppage time, proved the difference as Al Shabab Al Arabi secured a place in the Group Stage for just a second time.
Chonburi, Al Ettifaq and Neftchi will now take a place in the AFC Cup alongside Yemen's Al Tilal, who beat Maldivian side Victory Sports Club 4-3 in the play-offs on Saturday.