Hi, I've got a problem. Can anyone tell me who might have manufactured a "drop box" for a 1989 GMC C7500 TopKick. This truck was manufactured in late 1988 as a cab and chassis and I have called around to GM dealers and pretty much gotten laughed at for asking about something before 1990. This "drop box" is after the Allison automatic and resembles a transfercase without the output shaft to the front and uses an air driven shifter to switch ranges. This is a farm spreader truck for broadcasting Lime/Fertilizers and all of the ID tags are rusted away and lost. If there was ever an tag with option numbers for this truck I can not find them. Any information would be appreciated about what this gear box could be would be great.

Thanks