Need help identifying the engine in my 66 c10

[Jbarrkn493]

I just bought the truck (a 1966 c10), the seller said it had a mid-70s 350 small block. Ive looked up the partial vin and engine code but none of the numbers seem to match up.. Its running pretty rough, the carb is sticking, and it doesnt sound like its firing on all cylinders

Partial vin: C6J125936

Engine code: V1121TYZ

Any help would be greatly appreciated