the seat belt extenders are out there.
it is a seek and find job.

From GM

6" seat belt extender, part # 1253323
9" seat belt extender, part # 12514177 (black ?) (pn provided by "danstarley")
9" seat belt extender, part # 12514184 (tan ?)
15" seat extender, 1251-4191 or 12457414



also, 1980's Camaro seat belts are compatible replacements

source of Allanté seat belts

click here:
SEAT BELTS (http://www.usedpartslive.com/catalog/buy.php?make_text=cadillac&ml=alla--001&model_text=allante&year=1988&part_name=seat_be lt&brand_name=replacement&repcat=true&s=1)




http://www.usedpartslive.com/catalog/images/alloem_performance/thumbs200/replacement/US-SBELT-2.jpg