Is this a stock rebuild on your 257? Where you running mods before the rebuild? If so you may need to get a touch up tune.
As soon as my motor was finished I started data logging and sending the logs to my tuner. On 91 I was getting knock all over. On e85 it was okay at first but at around 800miles on the motor I started running lean all over the place, so my tuner got that fixed after that it has been smooth sailing.