Frank,
From you description, The tranny may be the SM-465. I have on in my 84 C20. Good tranny, generally sounds like it is chewing on rocks when in neutral and the clutch engaged. Chevy put that tranny in everything up to the C-60's. Visual from under truck, 2 PTO plates, and a total length of about 12 inches.
Given that you have a c-30, rear gear most likely will be a 4:10 . Like DmaxMav said, you could have a 4:56 back there, I think it was an option in those trucks. Rear axle is probably a Dana-70. Cover has 10 bolts and is a full floater.
HammerWerf
1999 k2500 Suburban Pyro, Tranny Temp, Boost, K&N Air Filter, +3.5 qt tranny pan. 190000 miles
1984 C20 249k miles DSG gear, SM-465, 3.73
1983 C30 CC dualie. 308K miles on Chassis, TH400, 4-45 A/C Looking for a TurboCharger