What is it? C20

[Benzdoctor]

This is my first post on this site. I think this site is the best thing since sliced bread. I just bought this truck this past weekend. Here is the story behind it. The guy who bought this truck new in 1971 gave it to his grandson about 3 or 4 years ago. The kid sold it to a buddy of his for 200 bucks. The kid who paid 200 for it gave it back the the grandson. These two arn't very smart, they sanded the pait off the truck and disconected the fuel tanks and other stupid stuff. The way I found it is that a guy my dad works with is the grandson's step dad. The grandson needed money so I got it cheep. What I do know is that it is a factory big block A/C 3/4 ton truck with 3 gas tanks. I was told its a Cheyenne but Im not sure now after seeing it. I am also not sure what engine is in it. I thought after 1970 Chevy started using the 402BB. But the glove box says 396BB. Is this because it has a January build date, and they used what was left? The glove box also states it is a Custom Sport Truck. I thought that after 1970 they did away with CST? I tried to take pictures but it didn't come out so I will list the opitions on the glove box door. I would like to find out just what all this thing is.

LEFT SIDE:

MODEL #CE20934
1B85AB BELT MOLDING
1F51AC FRT & REAR SHOCK AB
1G50AE HEAVY REAR SPRING
1L47AG 396 CU IN V-8 ENGIN
1N33BG COMFORTILT STEERING
1RP5AC 875 X 165 X 6 TIRES
1U63AF AM PUSHBUTTON RADIO
1Z84BZ CUSTOM SPORT TRUCK
1621AE SADDLE VINYL CUSTOM

RIGHT SIDE:

1A11AA TINTED GLASS
1C60CF 4-SEASON AIR CONDIT
1F59AA FRONT STABILIZER
1G60AB AUXILIARY SPRING
1M49AJ TURBO HYDRA-MATIC
1N40AG POWER STEERING
1RP6AC 875 X 165 X 8 TIRES
1Z53AC AMETER-OIL-TEMP GA
1464AA YELLOW/WHITE/L

I am assumeing that the reason the tires are listed twice is that its been up graded to 8 lug? I am also guessing that the helper springs under it are factory, the aux spring? The truck has 81,789 miles on it. The truck is very straight and solid. I have only found one spot on the bottom front R fender thats through. I spent the afternoon cleaning out the garage so I could get around it. I have not had time to go through the bed or cab. I want to paint it back to the factory yellow and white. Im going to keep it mostly stock with a few changes.
The other truck in the pics is my 1971 c10. Its mostly all orignal with just over 100K on it. Its very base with only a couple options. I will be getting rid of that one sometime due to lack of space to keep all my toys.
Any help would be great. Thanks.
Joe