VIN DeCODER HELP

[nez71]

I need some help decoding a VIN #CE234SZ71803 its a 1970 GMC with a Bigblock I think its a 402, or 396, but the little old lady I bought it from says its a 454c.u. her husband bought it new in 1970 its a Custom Model pickup with factory. A/C, PB, PS, Cruise Control. She only wanted 800.00 so i bought it and drove it 750 miles home no problems. Im trying to get the engine size and see if anyone can tell me about it by the VIN number i got the first 4 digits then it does not make sense from any of the decoders i found online? Can anyone figure it out, thanks!!