Serial # confusion

[7 Devils]

So after trying to study the internet for information on my 55 370 2 ton, I've got more questions than I started with.
My serial number is 371 R Y2843 as found in the cab of the truck.
I'm assuming it's a 370 series, but booklets and this is also indicate it's a "6400". The ID plate shows 18,000 GVWR. It has a 270 I6, and a two speed dually rear end.

Anyone willing to help to help a newbie out on what this truck is? (Please).

Also, does anyone else have a schematic that covers frame dimensions for this truck that they'd be willing to share?

Thanks in advance

3/28/18

So after more time spent trying to hunt down the info, I am left with dimensional and model info from Chevy data of the same year. I really don't know if they directly cross over. They Chevy data suggests a 6403 Chassis but the GVWR #s are off and my door tag clearly says 18,000lb, not 19,500.