Finally got the instrument cluster -speedo cable issue - solved. The vacuum actuator that controls the defrost door mounted on the 77 AC plenum, interfered with the 74 speedo cable location of the GT dash preventing full insertion of the cluster into the dash frame. Found one off of a Dodge Ram truck with AC that looked similar and had a longer actuator arm allowing for relocating of the interfering actuator. We fabbed a tin bracket to hold the newly relocated actuator.