Keep the big block. It wouldn't even been an option for me to think about, personally. I wouldn't trade a 396 for that 350, let alone a good running 454. The 454 is going to make the same power the small block is going to make with less effort at a more streetable rpm, all things being equal.
Plus I wouldn't want anyone's "performance" engine (even a friends) that was taken out to .060 over and has never been ran. I just don't have much faith in people, I suppose.