At this time in the life of State vehicles, GM probably didn't make more than standard bed lengths for domestic consumption. I'd say just track down a dually bed. I believe the only model available that year was the 8' bed. However, as a cab/chassis, you could put on a flatbed or service body, too.

A SRW bed on a dually is not attractive. <img src="/forums/images/graemlins/scared.gif" alt="" />