Just checked in the Robert Goebel autobio ‘Mustang Ace’,he confirmed that they did indeed completely fill the 85usg rear tank,He says that he would take off with main (wing) tanks selected,after take off he selected the rear tank and burned that down to approx 30usg at which point he emptied the drop tanks before reselecting the rear tank to empty that !
He also said that if too much fuel was left in the rear tank (C of G too far aft) the a/c would ‘tighten up’ in the turn and loss of control could follow.

rgds baz