I believe there is a plate below the heads, upper block area with a coolant passage in it. Maybe it allows cross flow of coolant from left bank of cylinders to right bank.
2007 Classic GMC D/A Ext. cab,s.b.,built 11-2006,Di*Priol boost and pyro gauges,GT3794VA turbo, Edge Insight CTS,Garmin GPS,MBRP catback exhaust,300* hose permanent fix tranny cooling lines,M/A pump rub kit,Prodigy bc,Reese 16,000 lb. slider 4-way tilt hitch.Luverne SS side steps,Truxedo bed cover, Duraflaps all 4 corners, Michelin LTX M/S 2 tires and Bilstein shocks.