The 87.5" belt is for dealer installed A/C. Never seen that before, honestly. 104.5 is for the Cobra that had different diameter water pump and crank pulleys. Most of the belts listed for a GT with factory installed air have sizes ranging from 105.8" to 106.3". I would recommend going with the size LMR gave you. Motorcraft part number for the specific belt is JK61063.
The K in the part numbers for belts mark it as a serpentine setup, the 6 immediately after denotes it as being a 6-rib belt, and the four numbers following is the belt length to one decimal place in either inches or millimeters (inches in this case): 106.3"