Depending on the type of King Air from time to time you may find a pilot in the left seat of a King Air who has a PIC type rating that says "SIC Required" on it, common in the 1900 and 350, not sure about the rest. In this case you could log the SIC time under Part 91. Also if you hold a type on the aircraft (PIC or SIC, as outlined in 61.5(b)(7)) then you can log all time as sole manipulator as PIC time though you are not the acting PIC. Any aircraft under 12.5 (King Air 90, not requiring a type AFAIK) you can log all sole manipulator time as PIC.

As far as logging it under 135 if lieu of an Autopilot, have you completed company training? Are you a listed pilot with the company? You can't just pull someone off the street and throw them in the right seat, pull the A/P CB and start logging time. It has to be in the company manual that you are using the "FO in lieu of AP" reg, otherwise it's not approved.