X heads that flow 250 on 416

I recall the 416 Mike built w close to 500/500 on iron heads and a comp xe274s cam. I imagine he worked some serious magic on them heads. My x heads are limited to .550 lift did not originally go to extreme because they were going to be going on a stock appearing 340. My exhaust side is lacking a little flow, but not real bad for heads that started at 235/150