Chevrolet Repair: Chevy truck engine number, chevrolet c10, gm dealer

Question
I have a 1981 chevrolet c10 truck with what we think is a small block 400. this is the serial number off of the engine 15z420607T1022csm. We think it maybe from a larger truck. Your help would be appreciated.
chris

Answer
The fastest and easiest way to solve this problem is look on the block at the back of the motor right above the tranny, there should be a casting number that should tell you what size(5.0,5.7,6.0 etc.) if not then I would say to call your local GM dealer and give them the number to use, they should be able to tell you what it came out of and the engine size.