DC-9

Yes, safe to say the C-130 is the largest.

Never heard about the 727, but a modified DC-9 was a defintite COD proposal as discussed on secretprojects at
http://www.secretprojects.co.uk/forum/index.php/topic,662.0.html

description of the C-9 is as follows. Nose gear moved 12.5 feet aft. Nose gear strut capable of extending to give the aircraft a 6-degree nose up attitude for takeoff. Local strengthening for the arresting hook which was positioned on the underside (externally) under the engine location. A large tail bumper added on the underside roughly in line with the exhaust end of the engine. Wing folding was explored with the fold being at either of two locations. On one, the folded span would be 58.5 foot. On the other the folded span would have been 47 foot. In flight refueling probe extending from the lower left fuselage angling up then leveling again ahead of the nose at a level roughly that of the lower edge of the windscreen. Pylon attach points about 12 foot inboard of each wing tip. These pylons could have held the “buddy” refueling system. Thrust reversers omitted and replaced with simple exhaust cones. Air-stairs omitted. — Paraphrased from Great Airliners Volume 4: McDonnell Douglas DC-9, Terry Waddington, 1998. Pgs. 81-82.