Depending on your wheel base you could have 2, 3 or 4 driveshafts.
What's your wheel base (first two numbers on your serial number plate)?
8A = 132
8B = 144
8C = 156
8D = 174
8E = 192
The carrier bearing supports have different length hangers to keep the angles right. Not knowing what the fire service installers did and exactly where they moved or put bearings your best bet might be to find your local big truck drive shaft/suspension company and talk to them. I don't think you want a drive shaft 90" long. Seems like GM kept their shafts under 60". The last drive shaft I had made cost about $80, so it's not worth finding a junk one unless you need the jokes, and even then what's your time worth?
Hope this helps,
Mike B
