305th Ammunition Train was part of the 80th (Blue Ridge)Division commmanded by MG Adelbert Cronkhite from it creation at Camp Lee VA in 1917 to its return to the states in June 1919. To my knowledge (My father served in 305th Engr Regt at tah time) there may not be any published history of that unit but the history of the Division is well documented - See www.80thdivision.com.
As for HQ Det 320 Service Batallion may be HQ Detatchment (Company)320th Infantry Regiment, 80th Division.
Where did your ancestors live prior to entering the Army? 320th Infantry originally consisted of draftees from the City of Pittsburgh PA, 305th Ammo Train from central PA