I just picked up another 454 engine myself and the numbers wouldn't come up on Mortec or anywhere either. Evidently Chevy used some different casting numbers toward the end of the 454 production that aren't listed for some reason.

Closest I could find to your numbers is this one on Mortec:

12561357...454.......96-up...4-bolt, Gen.VI


Do you have a four bolt main, roller cam, plastic timing cover engine? If so, it is a gen VI. That is what both of the 454's I have are. Even the brand new crate 454 we ordered to replace the one I picked up had numbers that were no where close to anything anywhere that I could look up. I think since your number is only two numbers off of what Mortec shows either you are wrong on the last digit or it is just a later run than the one Mortec lists.

In either case, you and I own the good versions from everything I can read. If you want more info, here is a good site:

http://books.google.com/books?id=rUm...20main&f=false

Don