The maximum cruise speed from the brochure is a True Airspeed value and is based on the highest TAS achieved with the engines operating at the maximum cruise power rating specified by the engine manufacturer, based on some altitude and load condition that is probably buried somewhere in the fine print (we could probably assume the 365 kt value is at max cruise power, max gross weight minus departure and climb fuel, at service ceiling, in ISA conditions).
Note that limitation speeds like Vmo or Vd are based on indicated/calibrated airspeed (CAS, what the pilot sees as a speed on the instruments) and are a function of the dynamic pressure conditions acting on the airframe, regardless of the actual velocity through the air mass (True Airspeed). They are generally a lower value, as TAS starts to exceed IAS as soon as you go above sea level, the difference getting bigger and bigger the higher you go.
So at the 28000 ft service ceiling, the indicated airspeed will only be about 230 kt when the Herc is cruising at a TAS of 365 kt on a standard day with a -40F temperature outside.
The speed limitations will be a margin above the indicated/calibrated cruise airspeed value; if, say, the Vd was 290kt (I'm just picking a number at random), at 28000 ft in standard conditions, that's a TAS of 450, but that's irrelevant for the purpose of the speed limitation because we're only interested in the IAS value insofar as that's what the pilot sees, and it's what represents the actual dynamic pressure effects of that speed for structural purposes (although where the limit is for flutter considerations, that are a function of TAS, the flutter TAS limit is converted to IAS in the Air Data Computer for the pilot to use).
