I haven't seen a 602 in a 124 with the radiator out but you might be able to get an impact wrench on the crank pulley bolt. You need a flywheel lock to torque the crank pulley so an option is to get the lock in place at the start and use a lever arm to break the torque if an impact wrench doesn't fit. Using a lever arm on an engine stand is no fun.

Jim, I've never done an oil pump chain with the pan in place. Are you saying it's possible? Looks pretty tight in a 603 from these angles -





Sixto
87 300D