There is no squarely replacement until B727-200Adv is available.
Replacement possiblities for DC-8-20, DC-8-3x, DC-8-4x, DC-8-5x:
B727-100 and B727-200
B727-200Adv (range like -100 but seats like -200)
Also there is no replacement available for DC-8-62/-63 in the whole game. DC-10-30(ER) is much bigger, B767 won't arrive until the game ends, same with B757. Also the first A300 variants have much less range.
Hope I'm wrong. If yes, please correct me.