AFAIk, its a toss bet. Cadillac's Series 75 and Series 60 Fleetwood. The Series 75 has the longest wheelbase in GM's line up and its used for factory assembled limousines (unlike the super stretch cut-and-stitch limos of today). The commerical chassis of Series 75 is a bit longer since it used for hearses and ambulances. The Series 60 is usually used for the Fleetwood sedan, which has a bit longer rear passenger cabin compared to the De Ville or Calais.