CD, the shop manual indicates the LeSabres were available that year with a 455 as an optional engine. The Custom and Limited names have more to do with the trim levels and are independent of the engine type. Look at the VIN of the car, 5th character, to determine if it was built with a 455. The code letters are on this site. Just go to the home page, click on Shop along the left side, then click on Engine Identification.
The production code number stamped on the block will tell if the engine in it is a 455 and also the year. The engine serial number, also stamped on the block will tell if the car still has the original engine in it. Details of locating and deciphering these codes are there with the VIN codes. If more help is needed, just post a note back here.
